All that growth will come off easier while the boat is still in the water. Once you haul it out and it starts drying, the task becomes much more difficult. If you can't locate a dive service to do it.... find a shallow area near shore with a firm bottom... rather than soft, goopy one ...a beach perhaps.. bring it in close enough, parallel to the shore, so you can stand in the shallow side with a long handled scraper.... finish one side and turn the boat around to get the other. If you're standing on the firm bottom it'll be easier to get power in your scraping... if you float in the water it's more difficult to anchor yourself.
Once you have the big stuff off.... go back with a plastic scouring pad and scrub until you can see the bottom paint or bare gel.
Now....……… go do some research on how to maintain a boat's bottom. Fresh or Salt water, all boats experience growth on the underwater areas. If you found this forum you should be able to find many, many articles on bottom maintenance.