Port Clinton is a good jumping off point, not sure on ramps in that area though.
depending on what kind of a boat ride you want, you might want to put in somewhere on the west side of detroit, too.
I think rafting on the public docks is limited to 4 deep. that's usually where the party is, though. There are private marinas on both sides of the public docks, as well as balls to tie off to in the bay with a free shuttle to the island.