According to the Wiki:

If a zombie is in water, and not in a boat, after 30 seconds it will begin converting into a drowned. The zombie will start to shake, similar to a zombie villager being cured, and after 15 seconds the zombie will be converted to a drowned. Once this process starts, it cannot be stopped.

Current behavior:

Zombies that are submerged in water convert immediately into drowned without the intermediate 15 second period. During this time, zombies in the vanilla game can be removed from water while still converting after 15 seconds, so long as the original 30 seconds submerged was met. With this bug, that can no longer occur.