Aquatic terrain is the least hospitable to most PCs, because they can't breathe there. Aquatic terrain doesn't offer the variety that land terrain does. The ocean floor holds many marvels, including undersea analogues of any of the terrain elements described earlier in this section, but if characters find themselves in the water because they were bull rushed off the deck of a pirate ship, the tall kelp beds hundreds of feet below them don't matter. Accordingly, these rules simply divide aquatic terrain into two categories: flowing water (such as streams and rivers) and non-flowing water (such as lakes and oceans).
Flowing Water: Large, placid rivers move at only a few miles per hour, so they function as still water for most purposes. But some rivers and streams are swifter; anything floating in them moves downstream at a speed of 10 to 40 feet per round. The fastest rapids send swimmers bobbing downstream at 60 to 90 feet per round. Fast rivers are always at least rough water (Swim DC 15), and whitewater rapids are stormy water (Swim DC 20). If a character is in moving water, move her downstream the indicated distance at the end of her turn. A character trying to maintain her position relative to the riverbank can spend some or all of her turn swimming upstream.
Swept Away: Characters swept away by a river moving 60 feet per round or faster must make DC 20 Swim checks every round to avoid going under. If a character gets a check result of 5 or more over the minimum necessary, she arrests her motion by catching a rock, tree limb, or bottom snag—she is no longer being carried along by the flow of the water. Escaping the rapids by reaching the bank requires three DC 20 Swim checks in a row. Characters arrested by a rock, limb, or snag can't escape under their own power unless they strike out into the water and attempt to swim their way clear. Other characters can rescue them as if they were trapped in quicksand (described in Marsh Terrain).
Non-Flowing Water: Lakes and oceans simply require a swim speed or successful Swim checks to move through (DC 10 in calm water, DC 15 in rough water, DC 20 in stormy water). Characters need a way to breathe if they're underwater; failing that, they risk drowning. When underwater, characters can move in any direction.
Stealth and Detection Underwater: How far you can see underwater depends on the water's clarity. As a guideline, creatures can see 4d8 × 10 feet if the water is clear, and 1d8 × 10 feet if it's murky. Moving water is always murky, unless it's in a particularly large, slow-moving river.
Invisibility: An invisible creature displaces water and leaves a visible, body-shaped “bubble” where the water was displaced. The creature still has concealment (20% miss chance), but not total concealment (50% miss chance).
Land-based creatures can have considerable difficulty when fighting in water. Water affects a creature's attack rolls, damage, and movement. In some cases a creature's opponents might get a bonus on attacks. The effects are summarized on Table: Combat Adjustments Underwater. They apply whenever a character is swimming, walking in chest-deep water, or walking along the bottom of a body of water.
1 Creatures flailing about in the water (usually because they failed their Swim checks) have a hard time fighting effectively. An off-balance creature loses its Dexterity bonus to Armor Class, and opponents gain a +2 bonus on attacks against it.
Ranged Attacks Underwater: Thrown weapons are ineffective underwater, even when launched from land. Attacks with other ranged weapons take a –2 penalty on attack rolls for every 5 feet of water they pass through, in addition to the normal penalties for range.
Attacks from Land: Characters swimming, floating, or treading water on the surface, or wading in water at least chest deep, have improved cover (+8 bonus to AC, +4 bonus on Reflex saves) from opponents on land. Land-bound opponents who have freedom of movement effects ignore this cover when making melee attacks against targets in the water. A completely submerged creature has total cover against opponents on land unless those opponents have freedom of movement effects. Magical effects are unaffected except for those that require attack rolls (which are treated like any other effects) and fire effects.
Fire: Nonmagical fire (including alchemist's fire) does not burn underwater. Spells or spell-like effects with the fire descriptor are ineffective underwater unless the caster makes a caster level check (DC 20 + spell level). If the check succeeds, the spell creates a bubble of steam instead of its usual fiery effect, but otherwise the spell works as described. A supernatural fire effect is ineffective underwater unless its description states otherwise. The surface of a body of water blocks line of effect for any fire spell. If the caster has made the caster level check to make the fire spell usable underwater, the surface still blocks the spell's line of effect.
Spellcasting Underwater: Casting spells while submerged can be difficult for those who cannot breathe underwater. A creature that cannot breathe water must make a concentration check (DC 15 + spell level) to cast a spell underwater (this is in addition to the caster level check to successfully cast a fire spell underwater). Creatures that can breathe water are unaffected and can cast spells normally. Some spells might function differently underwater, subject to GM discretion.
Note The rules for whirlpools were created by a Pathfinder Roleplaying Game 3rd Party Publisher. Consult your GM before using or referencing.
A whirlpool is a swirling body of water typically formed by ocean tides. Most whirlpools are not very powerful, nor particularly long lived. More potent ones are more appropriately termed maelstroms, and can be permanent fixtures of the sea. While very powerful whirlpools are created in narrow shallow straits with fast flowing water, the most powerful maelstrom are formed near portals to the Elemental Planes of Water.
Whirlpools come in one of four sizes: vortex (10 to 50 feet in diameter), standard whirlpool (51 to 100 feet in diameter), maelstrom (101 to 500 feet in diameter), and greater maelstrom (501 feet to 2,000 feet in diameter). Whirlpools are typically as deep as they are wide.
Whirlpools are surrounded by strong feeder currents that can draw in swimmers far from the whirlpool itself. These currents work like strong water currents that always move towards the center of the whirlpool. The speed of a feeder current immediately next to a maelstrom is 120 feet. This speed is divided in half at each range increment from the center of the maelstrom. Table 1-12 describes the length of each range increment based on the size of the whirlpool. The current is generally negligible if its speed is less than five feet.
For example, a feeder current that exists 950 feet from the center of a maelstrom moves at a speed of 15 feet, while a greater maelstrom's feeder currents at the same distance would still have a speed of 120 feet.
Once a swimmer is sucked into the whirlpool by the feeder currents rushing toward it, the victim becomes hopelessly trapped. During each round of being trapped the victim takes bludgeoning damage from being violently whipped around and battered.
Table: Whirlpool Statistics indicates the number of rounds trapped and the bludgeoning damage for each round that the victim is trapped, based on the size of the whirlpool.
A vortex can only trap objects or creatures of Huge size or smaller. Gargantuan-sized creatures or greater cannot be trapped by standard whirlpools.
Maelstroms and greater maelstroms can trap creatures or objects of any size.
At the end of being trapped, the victim is propelled at a speed of 120 feet through the bottom of the whirlpool. This could be the equivalent of being violently shoved to the sea floor, squeezed through a hole in the bottom of the ocean, or punted into the Elemental Planes of Water— depending on what the Gamemaster decides is at the bottom of this terrible peril.