When a zombie gets in the water, it starts shaking. After a minute or so, it becomes a drowned. In the Java Edition of Minecraft, you can hear a sound effect. But in the Bedrock Edition of minecraft, it simply becomes a drowned, without any kind of sounds. For example, when you get a piglin in the overworld, it starts shaking too. But when it's transformed, you cand hear a sound of a change. Or when a zombie villager is cured, even he does a sound. I hope everything helped out!