When diving in a marina, be sure you know how to test for stray current. I don't mean water current, either. Electrocution can really mess up your day. If you don't know what I'm talking about, stay out of the water in the marina. And don't rely on the marina operator or boat owner to know either.
I do marine maintenance, mostly involving galvanic protection and marine electrical. Marinas can be very dangerous places to dive. I do hop in the water occasionally, but only after checking the boat I'm working on and the nearby boats carefully.
That said, I don't know if you've ever swapped out a prop underwater, but be careful. On the bigger boats (40'+), they're pretty heavy and need to be tied off. It usually requires a prop removing tool as well, even for the smaller inboards and sailboats. And there are some boats that have to have the rudder removed before the prop. There's also the risk of dropping something into the muck. I disagree with Ron that a prop is in the same category as golf balls. The item recovery, on the other hand . . . .
Okay, I charge $50 to splash, and $100/hour after that. But I'm paying insurance and stuff too. And I can guarantee my work. I am also a member of ABYC and adhere to their standards for boat repairs/installations.
It doesn't sound like you're interested in getting in the way of a commercial outfit. But as Bob3 says, stay on the good side of the marina operator. You might consider sticking to the recovery and finding buoy anchors and stuff like that. (Watch out for buoy mooring maintenance, if the boat gets loose you might incur liability, not just from the person who you did the work for, either.) Be safe, and don't get in over your head
