Quick test done, I added the splashes to my sdl file and it seemed better but can't be 100% sure. Another weird thing is that I remember when armed merchant fired it's machine guns at me, I'm getting different bullet sparks sounds eventhough there is only one sound listed in the sdl-file. Could it be that it can read all the others too?! I don't know!
The main thing in my opinion improving those airplane gun watersplashes is that there are few bullet sparks included.