rowing - lens filter advice

Hi,
i was thinking about buying a filter only to save my front lens from scratches.

i understand it's not the smartest thing to pile up glasses on glasses esåecially when they are not necessary, but the lens in question is a sigma 50-500 4/3 mount, and i'm afraid it would be quite hard to find a replacement these days

most of the times i'm shooting i'm out on a motorboat, so i get some little spash on the front glass now and then, and i often need to clean it. so far it has no scratch or anything, and i'd like to keep it that way the longest possible... what should i use? considering the water and the environment in general, would a skylight filter be a good choice? or should i pick something else?

i know e5 isn't exactly the kind of camera you would use for sport shooting, but it has served quite well so far, it does pretty much everything i need.

thanks for your time