Island Hopping/Cave/Beach Photography experience

Hi all! My family and I will be having a vacation on the beach. Part of our itinerary is to go island hopping and some underground river cave exploration. I would ask for some tips from some people here:

What I got:

Canon 50D

Canon 10-22 + Kenko slim UV filter

Canon 24-70L + Hoya UV filter

Canon 70-300 IS USM + Canon protect filter

Canon 580EXII

Optech USA rain cover

Hoya Slim 77mm CIR POL

Manfrotto 190CXPro3 Tripod + 486 Head

For most of our trip we'll be riding a small boat so water would be everywhere. I decide on using 50D+ 10-22 + 580EXII + Optech rain cover + rubber bands to protect the body from water splashes. The tripod will be housed in a plastic cover to prevent water splashes during transport. Ofcourse I got a protective filter on top of the lens (which I will stack CIR POL during sunny hours, I don't care much about vignetting on 10mm, I'll just not use the widest end with a CIR POL on top). Problem is the salt water splashes on the filter. How do I clean it? Won't rubbing scratch the lens because of salt which are abbrasive?

For the underground river/cave exploration, my problem would be lighting. Yes, I could use my tripod to stay put but remember we're in a boat, so it moves. Any tips to get the best exposure? I don't have pocket wizards nor offshoe cord for the flash, so it stays on the 50D.

On the beach, should I remove my rain cover or should I keep it still on the camera and adjust with regards to lenses?

Any other tips?

Thanks in advance!