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Aquatic Rules

Source Starfinder #36: Professional Courtesy pg. 47
The following rules represent a compilation of and expansion on the rules for underwater adventure presented in various sections of the Starfinder Core Rulebook. Most of these rules relate to creatures that are completely submerged, except where noted otherwise.

Surviving Underwater

Source Starfinder #36: Professional Courtesy pg. 47
Most Starfinder adventures assume that the PCs wear armor which affords environmental protections for a number of days equal to its item level (expended in 1-hour increments) unless otherwise specified. These protections provide two primary benefits to a submersed creature: self-contained breathing and protection from the pressures of extreme depths, as described below. The space suit personal item also provides these protections.

Most adventures assume that NPCs have adapted to their native environments and therefore ignore many of the dangers and penalties that follow.

Suffocation and Drowning

Source Starfinder #36: Professional Courtesy pg. 47
Unless a creature has a way to breathe underwater (or doesn’t need to breathe), it must hold its breath or risk drowning. A creature can hold its breath for a number of rounds equal to twice its Constitution score. If the creature takes a standard or full action, the remaining duration that the creature can hold its breath is reduced by 1 round. After these rounds have elapsed, the creature must attempt a Constitution check (DC = 10 + 1 per previous check) each round to continue holding its breath. When the creature fails one of these Constitution checks, it begins to suffocate. On the first round of suffocation, it is reduced to 0 Hit Points and becomes unconscious (but stable). The following round, it is no longer stable and gains the dying condition. On the third round, the creature suffocates and dies.

An unconscious character must begin attempting Constitution checks immediately upon losing air access (or upon becoming unconscious, if the character was conscious when they lost access to air). Once the character fails one of these checks, they are immediately reduced to 0 Hit Points and gain the dying condition. On the following round, they suffocate and die.

Certain effects, such as androids’ constructed racial trait, the life bubble spell, and the water breathing universal creature rule allow creatures to mitigate or ignore this danger.

Extreme Depth

Source Starfinder #36: Professional Courtesy pg. 47
At certain depths, the pressure of the surrounding water becomes so great that it endangers non-adapted creatures (such as most PCs, including aquatic species).

A creature in deep water (100–999 feet deep) must succeed at a Fortitude save (DC = 15 + 1 per previous check) or gain the sickened condition. This condition ends when the creature returns to a normal depth.

Severe depths (1,000 feet or deeper) are far more dangerous. Every minute, a creature not accustomed to these depths that needs to breathe must succeed at a Fortitude save (DC = 15 + 1 per previous check) or begin to suffocate as though they’ve lost access to air (see Suffocation and Drowning above). In addition, every hour while at these depths, all such creatures without environmental protections must succeed at an additional Fortitude save (DC = 15 + 1 per previous check) or take 2d6 bludgeoning and 2d6 cold damage.

Note that even aquatic creatures and others able to breathe underwater (or who don’t breathe at all) are still subject to the non-suffocation effects described above.

Underwater Movement

Source Starfinder #36: Professional Courtesy pg. 47
Moving underwater can prove difficult for land-based creatures that don’t have a natural swim speed. Those with swim speeds (whether from their species, gear, magic, or some other effect) have distinct advantages.

Swimming Without a Swim Speed

Source Starfinder #36: Professional Courtesy pg. 47
A creature that takes a move action to move its speed while underwater must choose a direction to swim (including up or down) and attempt an Athletics check to swim. The DC of this check is 10 in calm water but increases in rough conditions (DC 15), stormy conditions (DC 20), and maelstrom conditions (DC 30). Strong currents, debris, and other environmental conditions can increase this DC further. A creature can’t take 10 to swim in combat or in stormy or maelstrom conditions, even if not distracted or in immediate danger.

On a successful check to swim, the creature moves half its land speed in a straight line in its chosen direction. If the creature wants to change direction (including ascending or descending) while swimming, it costs an additional 5 feet of movement to turn 45 degrees. See the Swimming Example below for an example of a creature swimming and changing direction.

If a creature fails a check to swim by 4 or less, it makes no progress. If it fails by 5 or more, it sinks, moving 5 feet directly downward.

Every hour while swimming, a creature without a swim speed must succeed at a DC 20 Athletics check or take 1d6 nonlethal damage from fatigue.

Swimming Example

Source Starfinder #36: Professional Courtesy pg. 47
Suppose a creature has a swim speed of 30 feet. With a successful check to swim (if required), it can swim forward 5 feet, turn 45 degrees to the left, and swim 5 feet diagonally, all of which costs a total of 15 feet of its movement. The creature can then turn 90 degrees to ascend 5 feet straight up, which costs another 15 feet of movement (10 feet to change its direction and 5 feet to move). At this point, it has used its 30 feet of swimming movement, so its move action ends.

Swimming With A Swim Speed

Source Starfinder #36: Professional Courtesy pg. 48
If a creature has a swim speed (whether from its species or another source), it can move that full swim speed when it takes a move action to move its speed. This movement uses the same rules presented in the Swimming Example below, which also provides an example of a creature swimming and changing direction.

A creature with a swim speed doesn’t need to attempt an Athletics check to swim except in hazardous circumstances, including stormy and maelstrom conditions. It also gains a +8 bonus to Athletics checks to swim and can always take 10 while swimming, even during combat and when distracted or in immediate danger. Creatures with a swim speed can also use the run action while swimming, provided that they swim in a straight line.

A creature with a swim speed (such as from their species) doesn’t need to attempt an Athletics check to avoid fatigue damage for each hour it swims.

Underwater Combat

Source Starfinder #36: Professional Courtesy pg. 48
Non-marine creatures often have considerable difficulty fighting in water. The following adjustments apply whenever a character walks in chest-deep water, becomes fully submerged, or traverses along the bottom of a body of water.

Attacks: Attacks made underwater take a –2 penalty. Attacks that deal electricity damage take a –4 penalty instead of a –2 penalty.

Attacks from above Water: Characters have cover against attacks originating from above water in the following situations: characters are fully immersed, or they’re swimming or floating at least chest-deep in water.

Thrown Weapons: When used underwater, the range of thrown weapons is reduced to a maximum of 5 feet; this limitation overrides any abilities that normally increase a thrown weapon’s range or range increment (such as the grenade expert ability of the soldier’s bombard fighting style).

Damage: Most attacks underwater deal half damage (minimum 1 damage). However, melee attacks that deal piercing damage deal full damage, and any attacks that deal fire damage (including spells that deal fire damage) do one-quarter damage rather than half damage.

Combat Maneuvers: The distance you move a target with a successful bull rush is reduced by 5 feet. The trip combat maneuver has no effect underwater.

Burning Condition: Effects that would apply the burning condition (such as the burn critical hit effect) instead deal the damage listed for the condition only once (the normal underwater reduction for fire damage still applies; see Damage above), and the creature doesn’t receive the burning condition.

Spells: Some spells work differently underwater, either as stated in their spell description or per the GM’s discretion. Spells with the fire descriptor often function differently when used underwater.

Prone: Creatures swimming in water can drop prone only if they’re standing on the bottom of a body of water. If a creature underwater would be knocked prone by an effect, it instead gains the flat-footed condition and takes a –2 penalty to attack rolls. These effects last until the affected creature takes a move action to reorient itself.

Stealth and Detection: It can be hard to find cover or concealment to hide underwater (except along the seafloor). How far a character can see underwater depends on the water’s clarity. As a guideline, creatures can see 4d8 × 100 feet if the water is clear and 1d8 × 10 feet in murky water. Running water always counts as murky unless it’s in a particularly large, slow-moving river.

Related Rules

Biomes (Source Starfinder Core Rulebook pg. 396)