Sounds like you want to use a Physics Volume - check out https://docs.unrealengine.com/en-us/Engine/Actors/Volumes/PhysicsVolumeDetails
Mark it as containing water then that should be what you’re after. I think there’s a few tutorials for this kicking around. Can’t help much more than that I’m afraid - I’m new to Unreal myself having moved from Unity.
Cheers,