Breathing	Movement	Vision	Combat	Underwater Spell Use
The Adventure	-		-	DMG

As all readers of fantasy know, the ocean floor is home to numerous ancient submarine civilizations and dark, green realms of creatures half-man && half-fish.
    <1 shade off?, headings? harmonize, in part, or in full, with sahuagin page>
Your players may have heard tales of the mountains of sunken boats that have been collected there over the centuries,
of such things as pearls the size of a man's head,
of beautiful mermaids with green eyes and blue skin . . .
If they should find some way to investigate these stories, how will you handle it?
This section deals with methods far conducting underwater scenarios.

Breathing:
The first major concern in underwater adventures is breathable air.
Magic-users have the advantage in this area, as they have access to several spells
that can solve the problem of submarine respiration.
These include water breathing (which is also a druidical spell), airy water, shape change, or even wish,
which might be made to work for the whole party (illusionists would use alter reality).
They can also polymorph others into forms that can breathe water.
However, most non-magic-users will probably find the use of magic items or potions safer ond more reliable.
These include potions of water breathing or items like the helm of underwater action or the cloak of the manta ray
(which help in movement as well as respiration).
The DM may also find it expedient to create such things as "air pills"
or seaweed herbs that confer the temporary power to breathe water when ingested.
Most methods of underwater breathing are of limited duration,
so most submarine adventures will be similar to dungeon adventures as regards time actually spent underwater.
Players will have to get to their destination,
accomplish their mission and return in a matter of turns,
rather than days or weeks as in other outdoor adventures.
If extended campaigns are desired,
the DM will have to ensure that the players can acquire some sort of equipment or comestible that will allow them to stay underwater for on unlimited time.

Movement:
There are two possible modes of movement underwater: swimming and walking.
Swimming is not possible in any type of armor heavier than leather (except magic armor),
or when encumbered with more thon 20 pounds of equipment of any type
(add or subtract 1 pound for each 100 g.p. worth of strength bonus or penalty).
Though submersion in water makes everything "weigh less" due to buoyancy,
actual mass of equipment is unchanged,
and the same density that causes the illusion of less weight also causes a resistance to movement that slows it down considerably. Therefore, movement (either swimming or walking) is the same as the speeds used in dungeons,
even though underwater movement is "outdoors".
Average movement is a function of encumbrance in exactly the same ratios as in dungeon movement.

Persons able to swim (due to lack of encumbrance, not innate natatorial ability) will be able to move vertically as well as horizontally, and at the same rate of speed. Remember that swimming persons ore vulnerable to attack from every direction.

Characters encumbered with more than about 20 pounds of gear will be forced to walk on the floor of the ocean,
lake, river, or whatever.
They will have to negotiate underwater hills, coral outcroppings, shipwrecks, seaweed forests, etc., in the normal manner, i.e.,
they will hove to go around or through them.
Characters equipped with a ring of free action or other magic item that confers the same power will be able to move normally and cover distance as if in the wilderness - that is, three times dungeon rate.

Vision:
Distance of vision will vary according to depth (available light) and obscuring obiects.
Basically, characters will be able to see objects and movement up to 50' away in fresh water and 100’ away in salt water.
The depth limit of vision is the same as the distance limit: characters can see until they go below 50' in fresh or 100' in salt water. Below this depth vision will be obscured.
You may wish to vary the distance as a function of depth to make this slightly more accurate,
so that characters in fresh water could see 50' at a depth of 10’, 40' at a depth of 20',
and so on in segments of 10' until vision is 0' at 60' of depth.
The same formula would apply to salt water, starting with 100’ of distance at 10' of depth.
Use of a light spell would allow vision up to 30' distance regardless of depth,
or add 10' of vision to any distance shorter than 60' (whichever is greater).
The visual capability of a helm of underwater action will quintuple normal vision as pertains to both distance and depth.

lnfravision and ultravision are useful underwater, and their distance limits are the same as in dungeon settings.
There are some problems, however:
infravision users may become confused due to shifting currents and layers
of different-temperatured water, as water exchanges heat more slowly than air and therefore is of a less even temperature.
Distance of ultravision is halved at 100' depth and reduced to zero below 200’ as ultraviolet "light" does not penetrate beyond that depth in sufficient quantities for sight.

Other objects will also obscure vision.
Seaweed or sea grass will reduce vision to 10' or perhaps nil for those within it, depending on its density.
Sea grass can be anywhere from 3' to 30' in height,
while seaweed can take practically any size or shape the DM desires.
In any case, shoals of either will totally obstruct vision, and anything may be hiding within!
Schools of fish can also blind and confuse with their masses and quick movements.
Mud can also be a big problem, especially as pertains to combat on the bottom, where violent movement may kick i t up in great clouds, totally blocking vision as long as the movement lasts and for 7-12 (d6 + 6) rounds afterwards, unless there is a current to carry it away.
Even light sources cannot penetrate the muddiest water.

Combat:

1.

Underwater combat is difficult for normal land-born characters.
* Due to water resistance, effective use of crushing or cleaving weapons will not be possible --
only thrusting weapons will be of any USE (this means spears,
tridents, daggers, stabbing swords, etc.).
* Human-types will also be somewhat
slowed underwater, so aquatic creatures will always get the first
chance to hit, unless the human is armed with a significantly longer
weapon than the opponent.

* Only those characters with free action ability
(from a ring, helm or otherwise) will be able to MOVE as if above water:
they can use any type of weapon they could normally USE, and they receive
no reaction penalty.

Nets:

Nets will prove useful as an adjunct to thrusting weapons underwater,
especially if both of the combatants are swimming. There are several ways
nets could be used.

Two or more characters could stretch a large net between them and foul
up attackers or combatants by maneuvering it so that their opponents become
entangled therein. This is most effective vs. charging or unsuspecting
opponents, who will have less chance of avoiding it.

Nets wielded single-handedly in combat must be weighted.
As the net is thrown, it is given a slight horizontal spin.
The spin causes centrifugal force to move the weighted ends out, which keeps the net open and untangled.
If thrown correctly, it will slow and stop spinning as it arrives at its target, the weights then pulling the net down over the victim.
Nets can be thrown one foot for every point of the thrower's strength.
Underwater races can throw nets an average of 15', sahuagin 20'.
Weighted nets can also be dropped upon non-swimmers on the ocean floor to drag them down or incapacitate them.
Some battle nets have little hooks or barbs attached at every intersection to ensure that targets will be unable to escape easily.

All of the undersea races use nets, particularly aquatic elves, locathah, and mermen; the most adept of all are the sahuagin.
They will often set ambushes involving many small nets or one huge net several hundred feet in diameter.
These may be concealed in seaweed or left floating near the surface, disguised as something else.
The sahuagin are masters of combat network as well, and will often fight with a net in one hand (to entangle and confuse) and a short, jabbing trident in the other.

The underwater combat net is a difficult weapon to use, and player characters who attempt it will do so at -4 to hit
(unless they undergo extensive training and choose the underwater net as a weapon of proficiency.
Such training must take place underwater).

Crossbows: Except for certain specially-made crossbows, use of missile weapons is generally impossible underwater.
Special crossbows which will function underwater can be made by knowledgeable bowyers for ten times the normal price.
Effective underwater range of these will be one-half normal (dungeon distances).

If characters intend to go somewhere where normal missile weapons could be of use (like the great air-filled domes of Atlantis),
bows and missiles must be kept dry. This is doubly true of such things as scrolls and books, of course.

Underwater spell use:

Spell USE underwater will be limited to the same ranges and distances as
in dungeons. In addition, there are problems with spell preparation, as
many material components will be altered by or will not work correctly
underwater. Some spells will be altered in effects as well. Fire-based
spells (such as fireball) will not function at all underwater (except within
the radius of an airy water spell). Electrical spells will be conducted to the
entire surrounding AREA -- a lightning bolt will behave as a fireball,
etc. As DM, you can alter whatever spell preparations or
effects you deem necessary and reasonable.