Version 0.25: This article may not be up to date for the latest stable release of Crawl.
An aquatic creature is one whose habitat consists solely of water or lava. Such creatures cannot willingly leave their environment, and if forced onto land will take 1-5 damage each turn. If an aquatic monster is currently flopping around on land, it cannot really control where it wants to move, but there is a 50% chance of flopping into an adjacent water (or lava) tile. The player can force aquatic creatures from water or lava by confusing them.
Because aquatic monsters usually cannot leave the body of liquid they inhabit, you can escape them by simply staying away from the water or lava tiles. Many of these creatures have powerful ranged attacks, however.