Imagine two different water images, fading in and out of each other. For example, image A is water in a certain position and state, and image B is also water, but in a different position/state. These could be superimposed upon each other, you don't even need a plugin for that, just use "Show Picture" and "Move Picture". One fades in as the other fades out. And then vice-versa. I haven't tried it (I just thought of it), but that might create a decent water effect.
How CPU intensive this will be I would think depends on what else is going on at the same time. But who knows? I'm always amazed at what things RMMV handles well, and what things make the framerate drop to 5. I feel like RMMV is very hard to predict, at least when it comes to performance (except that it generally sucks).