Are you really recommending an out of tower rent a canoe to make an unguided dive at HH?
Perhaps many people would do just fine, but seems like risk/effort/benefit is a little out of whack. Is there a good place to get in/out of a canoe with dive gear? I am always surprised and how few people can track a canoe straight, let alone right one that has capsized.
Crystal River might be considered a “sucky” dive by many who have been all over NCFL, but there are advantages. A guide taking you to snorkel at Sisters, and an easy dive in King Spring can be nice. The perimeter of King, might be the most likely place to see a manatee on scuba. ...which did seem to be the OPs stated goal.