Prey drowns when the predator is underwater
Prey players will still drown when positioned underwater. Normally prey should not drown when the predator is submerged, as it's implied that there's air in the stomach. Drowning should only be allowed if the predator is dead while underwater, in which case it makes sense to allow this to occur.
I haven't investigated whether there's a drowning timer exposed as a player variable, in which case a solution should be fairly simple. The best option though would be telling Xonotic that the prey is not underwater, which also prevents stuff like the underwater blur effect (HUD post-processing).