Dive yourself or hire someone....
....I'm in salt water and I use a wetsuit so I need a weight belt to keep from bobbing to the surface. I also loop a length of rope around the bottom, tied to stanchions, to give me something to hold on to. Use the least abrasive scrubber you need to do the job. If it's just algae, an old piece of carpet would probably work fine.If that is to much of a pain, hire a diver. Ask around the dock or check the yellow pages, expect to pay $30-40 for the first cleaning, $20-25 after that. One thing a diver will do is a complete inspection for things like electrolyis and blisters etc. You should have had that done when you purchased the boat, though.