We enjoy floating many of our local rivers and backwoods camping here in the ozarks, not usually anything to worry about but these days ya never know so I've always got one of my firearms in one of my drybags.
Think that I'd either have it lanyarded to my person, or have the bag hitched to the boat. If it gets wet, so what? Exterior should be chromed or Tennifered anyway, and all it takes to make the water "bead up" to be wiped off is a shot of WD-40. Oils and the car wax don't mix, tho.