You can bake out render targets directly from materials with blueprints, but I’ve only used it in that way for baking textures which didn’t need to have any sort of view projection.
Perhaps you could have an invisible camera aligned plane shaded with SLW. Then you could have it save it’s scene depth w/o water directly from it via bp like baking any texture. You could probably get away with doing it at less than screen resolution. If you need info on how to render a material out to a target with BP, Ryan Brucks has a tutorial that could give you a head start.