The following is which of the myriad rulebooks insight into adventuring underwater we will be using in our campaign. Since none of these sources take into account the use of subabilities, note here which subabilities are relevant for various checks. I'm not trying for style points here, just trying to recap the salient rules as succinctly as possible. Also, I've modified a couple of the rules based on common sense.

Surviving

Breathing

With a good gulp of air you can hold your breath for a number of rounds = one third CON/Health. If exerting yourself, reduce this time by half. Also reduce by half if you don't get a chance for a good gulp of air. (2ePH p.122) Always round up. Beyond this time, make CON/Fitness checks each round to continue not breathing. Each subsequent check is at -2 penalty. You have to breathe when you fail and if still underwater, you fall unconscious (0 hp). (3e DMG p. 85) You fall to -1 hp the next round and the following round you drown.

Note that using Water Breathing spells cause lungs/windpipe to fill with water making speech impossible. (S&S p. 67-8)

Temperature

Cold water: Constitution/Health check each hour submerged. Cumulative -2 penalty/hour. Failure indicates a loss of d4 Constitution points. If Con reaches 0, you freeze to death. You can regain CON points at a rate of 1/hour when resting in a warm, dry area.

Very Cold Water: Deals d6 subdual damage from hypothermia / minute of exposure. (3e DMG p.86)

Due to the cold temperatures below 100', non-magically protected surface dwellers suffer -2 on to hit & damage rolls as well as a +2 initiative penalty. This is cumulative with all other underwater penalties. (S&S p. 72)

Pressure

d6 damage / 100' depth / minute. Save vs. Paralyzation each minute to prevent damage that round. Initial save is at +4. A cumulative -1 penalty is added each round a character remains at that depth. (3e DMG p. 86)

Movement & Senses

Swimming

Nonproficient swimmers can swim underwater at (land movement rate/3 x 5 yards / round). This assumes non-encumbered status. Even a lightly encumbered nonproficient swimmer can not move at all. Only sink. In rough waters or strong currents, they may not be able to stay afloat. (2ePH p.121)

Proficient swimmers can move at (land movement rate/2 x 5 yards / round). If lightly encumbered they can move as nonproficient swimmers. Moderate and worse encumbered characters can not swim at all. Lightly encumberd swimmers have their movement rate halved. (2ePH p.121; WSG p. 41)

Proficient swimmers can double their swimming speed if they make a succesful STR/Stamina check. This requires a CON/Fitness check each turn to continue and avoid temp loss of 1 each STR & CON and -2 to attack rolls. This check is modified by -1 the first round, -2 the next, then -4, -8 and so on. (2ePH p.121)

You can add 1" to your base swimming speed for every CHARP you spend on the Swimming MWP after the initial cost. (S&P p.103)

Effects of Moving Water

Water currents will directly effect swimmers rate of movement. The rate will be added to their swim move rate. It is possible to be swimming as hard as you can yet still be pulled backwards by a rapid current. If there are hazards such as rocks or reefs in areas of rapid current, the area will have a danger rating of 1 - 10. Once / # = DR rounds the character will have to make a swimming proficiency check or take at least d6 damage from being buffeted on the rocks. As for nonproficient swimmers in these conditions, well, lets just say it can get really bad, really fast. (DSG p.14; WSG p.42)

Moving through extreme conditions, such as whirlpools and tidal waves is detailed under the spells of the same names in PFS p. 32.

Surfacing

All actively swimming characters can ascend at their max swim rate. Unconscious characters will either rise at 10'/round or sink depending on encumbrance.

Sinking

Sinking characters descend at a rate of 10'/round. Metal armor or carrying more than 30 lbs of stuff will increase this to 40'/round Swimming characters can add this sinking rate to their max swim move rate.

Diving

Any character can dive 20' in a round. A short run or some height above the surface will add 10' to this. for every 10' above the water, add 5' depth (max +20' depth). For each point of movement below your move rate due to encumbrance, add 2' of depth. (2ePH p.122)

Maneuverability

All surface dwellers must make a Dex check (Aim or Balance) when attempting abrupt or complex maneuvers underwater. Failure indicates they can not perform the action until the end of next round. Such actions include turning to face an enemy, grabbing for a spare weapon or coming to a full stop when moving at near top speed. Nonproficient swimmers suffer a -4 to the check. (S&S p. 78)

Floating

You can not float indefinitely. You still must tread water and are subject to fatigue. If all you do is float and/or tread water, you can maintain that for a number of turns = (CON/Fitness + your experience level) x 2. This assumes that you are unencumbered. (DSG p.13)

Endurance

If swimming at 1/2 normal speed and occassional resting by treading water - anyone can maintain for a number of hours = CON/Fitness After that, temp loss of 1 CON/hour. Make checks 1/hour to continue swimming. All of this assumes calm water. (2ePH p.121)

Swimming at normal movement rate requires a CON/Fitness check each hour, reduces CON & STR 1/hour and reduces to hit rolls by -2 each hour. (2ePH p.121)

If CON ever = 0, character drowns.

Resting underwater will only allow STR & CON to be regained at half normal rate. (S&S p. 78)

Walking

Assuming a solid bottom or underwater surface to walk on, characters can move at (land movement rate/3 x 5 yards / round). Walkers do not lose CON points as swimmers do and can turn to face an attacker with no penalty. Man sized creatures must carry at least 20 lbs of gear in order to stay grounded. (1e DMG p.56)

Visibility

See S&S p. 69-70 for charts with exact visibility ranges.

Infravision will not work below 25 ft. In the best of conditions, infravision is limited to 50% normal range. (S&S p. 69-71)

Magical light sources provide light, regardless of depth = being near the surface on a clear day.

Hearing

An unassisted surface dweller can only make out extremely simple speech. Concepts such as “go up”, “danger” and “flee”. And that only at a distance of 5 feet. Any speech trying to convey more complex concepts requires magical assistance. (S&S p. 73)

Combat

Underwater combat, as with any abrupt motions, may kick up sediment, sand or mud, depending on the ocean floor. This will generally clear in d6 + 6 rounds. (1e DMG p. 56)

Only thrusting weapons (type = Piercing) are useful. Slashing, Blunt, Thrown and Projectile weapons are all useless. Only specifically made crossbows function underwater. Nets are very effective. Net range = STR/Muscle in feet. (2eDMG p.79)

-4 to all melee initiative rolls (2eDMG p.79)

-4 on all attack rolls. (2eDMG p.79)

No bonuses to hit, damage or extra attacks/round due to weapon specializations. (S&S p.79) Certain magics can overcome this penalty.

Crossbows must save each turn spent underwater (cumulative -1 penalty) or become useless. In any event, their range is reduced by a factor of 10 and all damage is -1/die (and limited to base damage). (S&S p.80)

Weapons have adjusted stats underwater. Here's some of the ones commonly used by the party currently. (see complete chart, S&S p.82-85)

Spellcasting

Verbal Components

Impossible without magical help. Potions or spells of Water Breathing do not allow speech.

Somatic Components

Any spell requiring gestures has an initiative penalty of 4. Elementalist (water) wizards and priests of water gods (such as Jvelto) only suffer an initiative penalty of 2.

Material Components

These may become diluted or destroyed by exposure to water. Or they may simply float away when you're trying to cast the spell. The complete saving throw chart can be found at S&S p.89. Also see individual spell modifications for specific spell component saves on pages 94-102.

Impaired Senses

Being underwater impairs vision, hearing, speech and movement. See CWH p. 79-81 for a general discussion on these limitations.

Spell Modification

Basically, certain types of spells just won't work underwater. Spells involving fire, smoke, some liquids and powders fall into this category. Some spells behave differently, such as those involving electricity or extreme cold. Some spells work better, such as those involving sound or vibrations. (S&S p.90-102)