Aquatic

Most aquatic creatures have the aquatic subtype, meaning they breathe water. A creature that can breathe air and water has the aquatic subtype and the amphibious special ability. If an aquatic creature lacks the aquatic subtype, it has the hold breath special ability.

You can make aquatic creatures immune to the dangers of extreme depths without counting it against their special abilities.

You can add one sense for free if you wish. Among aquatic creatures, blindsense (scent, sound, or vibration) is common. Some such creatures have sonar, granting them blindsight (sound) out to 120 feet. A few aquatic predators have tracking (scent), normally representing the ability to sense blood in the water.

Athletics. The creature might have penalties to such checks on land.

Aquatic creatures normally have a swim speed based on their CR. This swim speed is typically 20 feet for creatures with a CR of 2 or less, 30 feet for CR 3–6, 40 feet for CR 7–9, 50 feet for CR 10–12, and 60 feet for CR 13 or higher (though you can make creatures faster or slower, based on the creature’s concept and other abilities). Many aquatic creatures have no other form of movement, though you can make flying, walking, or burrowing aquatic creatures if you wish.