I would do this with some creative EQ filtering on the primary audio track (mid-low pass randomized filters) and overlaying some water-like sounds. (I would stick a microphone underwater for those sounds.)
In theory-crafting, convolvers are primarily used for reverb, but I am wondering if a water sound track could be used instead of a typical impulse recording... Dunno. I have the tools for all of that, so it would be interesting for me to try that tonight, actually. (I am new to convolvers, so it may not be possible at all.)
Comment #2. So I played around a bit and got something muffled and a bit wobbly. It was basically mix of cropping out highs/mid-highs and adding various depth effects, mild hiss and some compression with actual water sounds that are playing super quiet. After all that mess, I EQ that again to get rid of any clipping or popping.
It's still not perfect but might be on the right track. I am in a road trip at the moment but I just randomized a piano and started slapping fx together. The fx are completely blown out on my phone speakers, but decent enough with headphones.
https://filebin.net/kdqlljwwwoknaeg5/water_sim.mp3
Here is what I am working with so far. You can see the fx chain in the right side. EQ -> Luxverb for depth -> Basic delay and reverb -> EQ #2 -> flange for a bit of hiss. (Track 5 hits the limiter on Track 1 for a bit of water-like wobble.) The first EQ low pass is purposely high. It creates a bit of distortion that kinda works.