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GM Core / Chapter 2: Building Games / Environment

Aquatic

Source GM Core pg. 90 2.0
Aquatic environments are among the most challenging for PCs short of other worlds and unusual planes. PCs in an aquatic environment need a way to breathe (typically a water breathing spell) and must usually Swim to move, though a PC who sinks to the bottom can walk awkwardly, using the rules for greater difficult terrain. Characters in aquatic environments make frequent use of the rules for aquatic combat (page 30, Player Core 437) and the drowning and suffocation rules (Player Core 437).

Currents and Flowing Water

Source GM Core pg. 91 2.0
Ocean currents, flowing rivers, and similar moving water are difficult terrain or greater difficult terrain (depending on the speed of the water) for a creature Swimming against the current. At the end of a creature’s turn, it moves a certain distance depending on the current’s speed. For instance, a 10-foot current moves a creature 10 feet in the current’s direction at the end of that creature’s turn.

Underwater Visibility

Source GM Core pg. 91 2.0
It’s much harder to see things at a distance underwater than it is on land, and it’s particularly difficult if the water is murky or full of particles. In pure water, the maximum visual range is roughly 240 feet to see a small object, and in murky water, visibility can be reduced to only 10 feet or even less.