Underwater

Swimming

[link href="http://paizo.com/pathfinderRPG/prd/skills/swim.html#_swim"]PCB Swim Skill[/link]

Roll a Swim check once per round while you are in the water. Success means you may swim at up to half your speed (as a full-round action) or at a quarter of your speed (as a move action). If you fail by 4 or less, you make no progress. If you fail by 5 or more, you go underwater.

A character with Swim speeds can swim, hustle, or 'run' per normal rules.

Alternately, the PC can be towed by another person using the Encumbrance Rules for Dragging.

Drowning

If a hero doesn't have (or loses ability) to breath underwater) he may hold his breath for #rounds = x2CON. After this, the hero makes DC 10 Con +1 per round check in order to continue holding her breath. When a hero fails his Con check, he begins to drown. 1st round, unconscious (0 hp). 2nd round, --1 hit points and is dying. 3rd round, he drowns (dies).

Very Deep deals water pressure damage of 1d6 points per minute for every 100 feet the character is below the surface. A successful Fortitude save (DC 15, +1 for each previous check) means the diver takes no damage in that minute. Descending is a Fort DC15 for one minute, and d6 damage for a failed save. At the Triton Palace, it is Fort DC 16 +1 per minute, and 2d6 damage for each fail. Any form with (acquatic) template is immune to the very deep damage.

Light

  At 0 -- 100 feet, can see100 feet
  At 100 -- 200 feet, can see 40 feet
  At 200 feet, the light is shadowed, can only see 20 feet.
  At 300 feet, it is dark.
  

Cold

Deep water and cold water bring additional dangers of cold weather exposure.

The Plactik Ocean is a [b]Cold Weather[/b] environment at 50 feet depth or below. Oceanic localized conditions may cause a change in this temperature as determined by the DM. Local conditions for smaller bodies of water are determined by the Board DM.

....

(PCB - Cold Dangers) Cold and exposure deal nonlethal damage to the victim. A character cannot recover from the damage dealt by a cold environment until she gets out of the cold and warms up again. Once a character has taken an amount of nonlethal damage equal to her total hit points, any further damage from a cold environment is lethal damage.

An unprotected character in cold weather (below 40° F)must make a Fortitude save each hour (DC 15, +1 per previous check) or take 1d6 points of nonlethal damage. A character who has the Survival skill may receive a bonus on this saving throw and might be able to apply this bonus to other characters as well (see the skill description).

In conditions of severe cold or exposure (below 0° F), an unprotected character must make a Fortitude save once every 10 minutes (DC 15, +1 per previous check), taking 1d6 points of nonlethal damage on each failed save. A character who has the Survival skill may receive a bonus on this saving throw and might be able to apply this bonus to other characters as well. Characters wearing a cold weather outfit only need check once per hour for cold and exposure damage.

A character who takes any nonlethal damage from cold or exposure is beset by frostbite or hypothermia (treat her as fatigued). These penalties end when the character recovers the nonlethal damage she took from the cold and exposure.

Aquatic Terrain (Pathfinder Core Book)

[link href="http://paizo.com/pathfinderRPG/prd/environment.html#aquatic-terrain-103"] PCB Aquatic Terrain[/link]

This link covers:

-Flowing Water

-Swept Away

-Non-Flowing Water

-Stealth and Detection Underwater

-Invisibility

-Underwater Combat

-Ranged Attacks Underwater

-Attacks from Land

-Fire Underwater

-Spellcasting Underwater

-Floods