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All Rules in Aquatic Terrain

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Underwater Combat

Source PRPG Core Rulebook pg. 432
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 13–7. They apply whenever a character is swimming, walking in chest-deep water, or walking along the bottom of a body of water.

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.

Table 13-7: Combat Adjustments Underwater

Attack/Damage
Slashing or BludgeoningPiercingMovementOff Balance?1
Freedom of movementnormal/normalnormal/normalnormalNo
Has a swim speed-2/halfnormalnormalNo
Successful Swim check-2/half2normalquarter or half3No
Firm footing4-2/half2normalhalfNo
None of the above-2/half2-2/halfnormalYes
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.
2 A creature without freedom of movement effects or a swim speed makes grapple checks underwater at a –2 penalty, but deals damage normally when grappling.
3 A successful Swim check lets a creature move one-quarter its speed as a move action or one-half its speed as a full-round action.
4 Creatures have firm footing when walking along the bottom, braced against a ship’s hull, or the like. A creature can only walk along the bottom if it wears or carries enough gear to weigh itself down: at least 16 pounds for Medium creatures, twice that for each size category larger than Medium, and half that for each size category smaller than Medium.