You are doing the best you can with what you have with that very low transom.
It really looks like a 20" standard transom was simply cut down for a 15 motor. OR the factory chopped down the inboard motor transom. There should be a...Splash Tray.... enclosure. All the way up to the top of the rear deck. That will stop splashing water into the hull.
But You will still have that stern down look. Because the hull is basically designed for the big heavy inboards normally installed much farther forward in the hull.
Boxing in the transom and motor will stop the water coming in over the low transom. You must never get caught in rough waves and a dead engine. You would need life jackets for every one. I would sell the boat. You now know what is bad about ...Cut Down Transoms. Good luck