Your water mesh probably isn’t thick enough to affect the mesh distance field, so you’re left with some options; you can check your player location, and if it’s Z location is lower than your water plane mesh around the vicinity where your water plane mesh is, you can toggle the rain particles off. Or, you can create a large cube mesh (with no collisions) with a masked material with a 0 constant plugged into the Opacity Mask node and line up the top of it with your water plane mesh. This will act as a “culling volume” which you can place, but you need to make sure that you’re scaling the cube uniformly, otherwise the mesh distance field will have “holes” in it.
Cheers!