Purely from observations as a scuba diver (who has done many dives when it is raining), I would say that the majority of marine life simply don’t care if it’s raining. So no change to their behavior.
As pointed out, I think a lot of marine life are “aware” something is happening, but other than possibly detecting a change in the salinity at the surface, I doubt they notice much difference in the environment.
Are raindrops likely to knock insects out of the air? Would pond fist be likely to cruise by the surface to look for them?
I have a small outdoor pond. When I add water, I set my hose head to shower and arc the spray up so it comes down similar to rain. (Does less damage to the plant life.) And the various fish do come to the surface when I do this.
I’ve done a bit of diving during thunderstorms and in my experience, once you get thirty feet or so down you don’t notice the noise of the rain or even see the flashes of lightning.