I’m not exactly sure what you’re asking, but yes, rain does have an effect on many aspects of sea life. Like everything to do with oceans, it has multiple effects.
”[Rain] affects the heat, momentum, water and gas exchange between the atmosphere and oceans,” said Dr. Jeffrey Nystuen, University of Washington oceanographer and research team member for NASA’s Tropical Rainfall Measuring Mission
Scientists Listen to Rain for Clues to Climate Change
If nature were to do something for a reason, it’d have to have a consciousness, would it not? There’s no reason for any of it. Just inter-related happenings which affect each other. I mean, you could say there’s a reason as to why rain falls (because of moisture in the air), and you could say there’s a reason why rain falls on the oceans (clouds above the oceans), but to say it rains in the ocean for a reason – to benefit sea life suggests Nature has made a concious decision to do something (the way I’ve interpreted it at least).