Currently, the sounds of waterfalls\cascades are the same both above and underwater. Since Minecraft adds some changes to the sounds when the player is underwater (which improves immersion), my suggestion is to make this mod's sounds change when the player is underwater.
Here are video examples of how I would imagine that would work (I've adjusted the sound in an external audio editor software):
Before
After
I don't know how vanilla Minecraft changes the sounds underwater - applies an audio filter to the sounds, or plays back separate "underwater" sounds - but could something like this be done for this mod?

Minecraft doesn't change sounds underwater, the only instance I can think of that does that is guardians with separated sounds. The rest is played just the same.

If you want more immersion like this I recommend Sound Physics Remastered, I took special care to make sure Effective is compatible with it.

I did try it with the Sound Physics mod and it works great! Thanks for the suggestion.