Water plugin and Sprite Niagara Particle Emitters rendering

I’m trying to simulate the cavitation bubbles streaming from a propeller: I tought to use a very simple strategy using a customized fountain particle emitter (with particle creation rate driven by propeller rpm).
I’m having some troubles emittin bubbles underwater (using standard Water plugin on UE4.27).
Everything works fine unless I put the propeller underwater … here particles are not rendered I can only notice the side effect of the emissive color on propeller surfaces but as long as I stay underwater the particels are not visibile… I noticed that the DefaultSpriteMaterial is Additive and tried changing it into Opaque: this seems to works since it make particles visibile but unluckly all the sprite canvas (?) is rendered (i.e. I got a number of boxes floating around instead bubbles) I got a similar behaviour using an emissive color setting into particle ScaleColor setting in Niagara emitter itself.
How can I fix this? How can I prevent sprites from being rendered out of water?

For the second part, if Simply use world position offset B as a mask that matches the height of the water body. In the material.

As to the first part.
Not sure, but I’d look at the post process effect for starters.