Deep Water
Lakes and oceans simply require a swim speed or successful Athletics checks to move through (typically, DC 10 in calm water, DC 15 in rough water, DC 20 in stormy water, and DC 30 in maelstrom water). Characters need a way to breathe if they’re underwater; lacking that, they risk drowning. When underwater, characters can move in any direction, including up and down.
Source
Core Rulebook pg. 396
