Anything from a zip-lock type bag (double zips may be safer) to an Otter Box to a Pelican case.
This may be one of those times when you will not feel embarassed to be seen wearing a lanyard.
If you are thinking you will get more than a bit of light spray you might want to avoid leather, although if done properly you can soak them in saltwater and bring them back to life. My recommendation is kydex over nylon just because kydex does not hold any water while most nylon holsters have a foam middle that will function as a sponge if gotten wet.
Speaking of spray - it's your call to oil down/spray the gun for "added protection" prior to the trip, but remember to clean it up when you get home.
Remember to wear your PFD (and helmet if going near rocks).