The new audio engine has lowpass submixes (amongst other effects) that work really well. Mix in with some audio ducking layered with underwater sfx I think you could get something pretty interesting.
Documentation isn’t great but there’s some in original New Audio Engine thread.
https://forums.unrealengine.com/development-discussion/audio/116874-new-audio-engine-early-access-quick-start-guide
Here’s a nice ducking tutorial Mixing Non-Linear Audio in Unreal Engine Using Sound Classes and Ducking - YouTube
Don’t have any hard and fast rules for you. I’ve never done this so I’d take me some experimentation. I don’t think ducking+low band pass will be enough. I personally would use those plus layer on some pre-designed under water audio on top.
That will depend on what you’re trying to achieve - a deep sea dive would be very different some a swimming pool.