Baldurs_Underdark
Hero
Slightly off topic
If they've been down for a long time, fish will eat the flesh and you get barnacle-covered skeletons, with an anemone-armor (you take poison damage if you touch it or hit with melee attack).
On topic
Any airpockets that a zombie has (e.g. whatever is left of the lungs) will certainly have leaks, so air will escape and they will sink.
Drowning won't make them any more dead, so they will just slowly walk along the bed, maybe bobble a bit, and perhaps stab the nearest lobster.
If they've been down for a long time, fish will eat the flesh and you get barnacle-covered skeletons, with an anemone-armor (you take poison damage if you touch it or hit with melee attack).
On topic
Any airpockets that a zombie has (e.g. whatever is left of the lungs) will certainly have leaks, so air will escape and they will sink.
Drowning won't make them any more dead, so they will just slowly walk along the bed, maybe bobble a bit, and perhaps stab the nearest lobster.