I have never tried a water repellant chemical. I am not 100% sure my window is not coated. So, not keen about smearing it regularly with a glycol ether (RainX is a mixture of 90+%water, isopropyl alcohol, and 2-butoxyethanol [Ref:
MSDS]).
What I found (with my Olympus TG-5, a small/thin compact):
1. Prevention: I try to keep my camera as warm as possible by keeping it against my chest, away from wind and spray (to minimize cooling, hence condensation, and direct salt water contamination) between PFD and wetsuit, lens toward body (camera is tethered to PFD). The other advantage is that it keeps the battery happy.
2. Monitoring: I try to systematically check the window for droplets or fog when I pull out the cam. It's an habit now.
3. Cure: 1 or 2 quick tongue wipes! Not perfect, but most of my pics have to be taken very quickly. No time to pull out lens cleaning gear. It does work quite well for me.
Whitish deposit on window? This is salt cristals → Use
very moist tongue, no pressure or a couple of drops from water bottle. Camera probably need a little fresh-water shower as well.
I still carry easily accessible lens cleaning material . If it's too messy, I stop and clean.