Thanks everyone for the replies. At least I don’t feel crazy for assuming $350 was a flat fee (I can’t say for sure exactly what he said on the phone when we set it up). To answer a few of the questions:
The hull was in typical condition. We use it a lot and I had not noticed any decrease in performance. He also confirmed that there was not excessive growth.
The location might be a thing that raises the price. I am in Napa, CA and the boat was at a Marina, but we have a hard time getting divers here. There was one diver that had a good number of clients but ha hasn’t been showing up lately. Most Bay Area divers won’t come up this far. I’m guessing that even though the new diver I tried out is advertising at the Marina, he does not have a bunch of boats to group together in one trip.
Moving forward, I think I am going to DIY it, we’ll see how that goes. I have been a recreational diver for over 35 years. I’ve cleaned the bottom myself a few times with just the partial tank left after a fun dive. I have not done the zincs though, because I figure that the number I drop in the muck will be incredibly frustrating. It seems worth a try now.