I was running around on a large beach biome, nowhere near the water, with my surfboard in inventory and I noticed that bits were breaking off and then submerging in bubbles in the sand, the way they do when your board is taking damage in the water. I don't know if it affected the board's actual health because I don't remember the value from before I picked it up, but it was already fairly damaged from use.
You can just see a piece vanishing under the tip of the picked sapling above Walani, with water drops - sorry it isn't clearer, they vanish quite fast before you can get a screenshot in. Yes that is fairly near the water, but it also happened much further away, this is just where I was able to get a picture. (This is either the day before or the first day of Hurricane season, by the way, and it isn't raining.)
Well that's going to be a difficult one, it's never happened to me before! Put surfboard in inventory slot, run around on sand, see if it happens again!