You'll want someone to help you with it if you DIY. Even if you do get the yard to do it make sure everything is well prepared, all tools , caulk and parts available and you've cleared the area around the through hull inside, hoses removed etc. Replacing the thru hull is quick, preparing will take some time. Letting the yard do it in the slings may end up being cheaper. As I recall, the hardest part was getting the hose off and on. I've done it myself, but like you say, I get a "free" haul out every year and I've had bad experience hiring out work. This is not an area that I trust the local "profesionals".