Movement: Creatures with a swim speed can use it to move freely underwater. Creatures without a swim speed who succeed at a Swim check can move up to a quarter of their speed as a move action or half of their speed as a full-round action. The DC of this check is 10 unless otherwise stated. If the creature has a surface to walk along, such as the bottom of a body of water, it can move at half speed without a check.
Melee Attacks: Natural attacks or attacks with bludgeoning and slashing weapons take a –2 penalty and deal half damage. Attacks that deal piercing damage don’t take these penalties.
Ranged Attacks: Thrown weapons are completely ineffective underwater. Attacks with ranged weapons except underwater crossbows (Pathfinder RPG Ultimate Equipment 17) take a –2 penalty on attack rolls for every 5 feet of water they pass through, in addition to the normal penalties
from range.
Spellcasting: A creature that cannot breathe water must succeed at a concentration check (DC = 15 + spell level) to cast a spell underwater.
Fire: Nonmagical fire (including alchemist’s fire) does not burn underwater. Spells and spell-like abilities with the fire descriptor
are not effective underwater unless the caster succeeds at a caster level check (DC = 20 + spell level). If the check is successful, the spell produces a bubble of steam instead of its usual effect but otherwise functions as described. A supernatural fire effect is not
effective underwater unless its description states otherwise.
Freedom of Movement: The spell freedom of movement negates the penalty on melee attack rolls with bludgeoning and slashing weapons and allows creatures to move freely in all directions as if they had a swim speed equal to their base speed. Such creatures automatically succeed at all Swim checks.
Invisibility: An invisible creature displaces water and leaves a visible, body-shaped absence of water where it is located. The creature gains concealment (20% miss chance) rather than total concealment (50% miss chance).
