This probably won't be useful for a while but I think an event like the one in this image would achieve what you had in mind.
I believe this would deal damage every 4 seconds the player is in water, give or take 1/20th of a second. It would deal damage faster if for some reason the player drank milk to remove slowness, but covering that would probably make this more complex than you want an On Player Tick event to be.
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This probably won't be useful for a while but I think an event like the one in this image would achieve what you had in mind.
I believe this would deal damage every 4 seconds the player is in water, give or take 1/20th of a second. It would deal damage faster if for some reason the player drank milk to remove slowness, but covering that would probably make this more complex than you want an On Player Tick event to be.
Huh, looks like the image embedder is broken. intended link is https://ibb.co/93mXRBk