Drown

From NexusWiki

If a character is swimming or otherwise in the water and the fifteen minute "swim timer" runs out, the character will drown, and receive the message:

"You have grown too tired and are unable to tread water any longer. You choke and gasp for air as the waves pull you under. You drown."

This is an instantaneous death. The character cannot avoid this save by continuing to swim (thus resetting the swim timer) or leaving the water. Characters with the Water Breathing skill can still drown if they run out of AP in the water.