For the most part, combat underwater isn't too different - but there are some exceptions. Only thrust weapons can be used effectively, unless you have a magical weapon with the power of Free Action. All forms of hurled weapons, except nets, are useless. Properly weighted nets are effective if used proficiently, but only have a range of 1' for every point of Strength possessed by the thrower. Specially made crossbows can be used underwater, but their range is halved.
Surface-dwellers receive a -4 penalty to both initiative and attack rolls while they are underwater, due to the slowness and the fact that they are unused to underwater combat.
Visibility underwater can vary depending on the weather, time of day, and - of course - your depth below the surface. It also varies based on water: in fresh water, which has more algae to obscure vision, you can only see half as far. The values below describe the visibility on the surface of the sea:
|Bright Sunlight||200 feet|
|Light Rain||100 feet|
|Sunset / Heavy Rain||50 feet|
|Full Moon||10 feet|
The first 600 feet below the surface is the "sunlight" zone. In this zone, visibility is as above. Below 600 feet, one enters the "twilight" zone. For every 100 feet you descend, visibility is reduced by 10 feet. For example, on a bright sunny day you could see 190 feet at a depth of 700', 180 at a depth of 800', and so on. At a depth of 2500', you would only be able to see 10 feet around you.
If you descend below the point where visibility is reduced to 10 feet, visibility reduces by 1 foot every additional 100' of depth. The official start of the "midnight" zone is at a depth of 3200', although depending on the light conditions the point at which it actually becomes pitch black is variable. In the midnight zone there is no light of any kind whatsoever, barring bioluminescence.
Untrained swimmers can manage to stay afloat if the waters are relatively calm and they are not encumbered (slowed). If encumbered, they will sink; likewise, if the waters are rough, they must make a STR check each round to stay afloat. Under no circumstances can an untrained swimmer do anything but keep their head above the water.
Proficient swimmers can swim, dive and surface. Their movement speed is equal to to 150% of their land speed - so a human with a speed of 12 has a swim speed of 18, and can move 180 feet in combat, or 540 feet if hurrying. Note that normal movement options like charging and sprinting are not possible when swimming. They can swim while encumbered, unless they have been reduced to 1/3rd of their original speed. In this case, their weight drags them under - though they can still walk on the bottom if this is an option.
A swimmer can dive at 20 feet per round - 30 feet if they dive into the water from a running start or short drop. For a longer drop, add 5 feet of depth per 10 feet of the dive. This only applies to the first round, of course. Each level of encumbrance adds 2 feet to diving speed. Swimmers can surface at a rate of 20 feet per round, but every level of encumbrance reduces this by 2 feet. If you are unconscious, you surface at a rate of 15 feet per round.
Although sprinting in the traditional sense is not possible, a swimmer can move more quickly when they need to. A successful STR check is all it takes to move at double speed for a round.
If they are careful and pace themselves, a trained swimmer can travel for a long time. If moving at half speed, you may travel for a number of hours equal to your CON score. After this point, there is a cumulative -1 to attack rolls per hour, CON will begin to drop at a rate of 1 point per hour, and a fresh CON check must be made at the end of each hour. Failure means the swimmer must tread water for half an hour before continuing. If their CON (or STR) reaches 0, they will begin to drown.
If a swimmer wishes to move at their normal speed, the effects are much the same. However, the CON checks begin after the first hour. Furthermore, both STR and CON are lost, and the attack roll penalty per hour increases to -2. Travelling at double speed is even more strenuous; it is the same as travelling at normal speed, but everything listed above applies every turn, rather than every hour.
Each day of rest restores 1d6 ability points and 2d6 points of attack penalties from swimming.
Holding Your Breath
Under normal circumstances, a character can hold his breath for a number of rounds equal to 1/3rd of their CON score, rounded up. If they are encumbered, were forced unexpectedly underwater, or are exerting themselves (i.e. in combat), this is halved. All characters are able to hold their breath for 1 round, regardless of circumstances.
Beyond this period, the character must make a CON check, with a cumulative -2 penalty each round after the first. If this check is failed, they must breathe and begin to suffocate if they cannot. Characters are helpless while drowning, but they can be saved if they are brought to the surface within 1 round. Characters saved from drowning are exhausted (-8 to everything) until they rest.