You'll have a problem getting most "smart" divers in the water to clean the hull on a boat while it's in a marina....
why? Because Marina's are known to not meet electrical codes and have GFCI breakers on every circuit, power cables fall in water, short out, etc. A diver swimming into that "electrical field" is pretty helpless as he gets electrocuted underwater.
Even if you cut the power to the main dock and put a padlock on it, someone would be running their generator and it could have the same problems. Boat owners also are known for not maintaining their equipment just the same.....
Then add to the fact that most marinas still have boaters 'dump' sewage in them, even though they aren't supposed to.
It'e no wonder you're having trouble finding someone.
With that being said, there are plenty of dumb rednecks who'll do this type of work still that dont' think about any of this....