That will take its toll on the leather. Keep in mind that even though it is leather, it is really cheap, poor quality leather. I'm not taking a shot at you or your truck but, I have yet to see good leather in a domestic vehicle. It would be best to try and put a towel down before sitting, though I usually load the boat and pull it out of the way with a wet suit too. I am also pretty sure that the back seat is vinyl that looks like leather, so the front seats are the only real concern.
I have been using Lexol products for years on my leather seats. Get the larger sized cleaner and conditioner. I would say that once a month you should clean and condition the seating surfaces (the only leather part). I have a 1995 Mercedes truck that did boat duty for years. I have used lexol on it and the seats look better than the new ones in my wifes '03 Chevy.