A lot of times fly reels get submerged or put on the bank, either or. In terms of potential damage, it could be possible for the drag to slip as some dont like getting wet. Most should be fine if it is a sealed drag system. If it is a click system the reel will be fine, just has to be dried thoroughly. Another issue with that is silt getting into the reel, which can cause problems depending where it goes.
I am not a gear guy so someone else will have to explain submersion with spinning and pin reels.
As far as saltwater use goes, always rinse/soak your gear afterward. The salt can corrode parts of the reel, affecting function. I know Lamson had a maintenance issue with their reels, and had to change instructions due to corrosion of some important bit. as a result they bucked up and covered the cost of part replacement.
hope this helps.