Yes, I talked to them about this and they do make you go in a group. You might be able to find an independent rental company on Cozumel that would let you rent a Harley on your own, but I don't know what the cost would be. It's worth looking into some more.
SMF, the Hogs on the High Seas cruise is something different than what corkgirl is talking about. The HOTHS is a special group cruise where they have pre-arranged for rental bikes (they don't bring their own unless they are part of the vendors on board) It would be fun to do a biker's cruise but it does cost a bit more than a standard cruise. Corkgirl was talking about just going on a tour like the jeep excursions only with Harleys. Corkgirl, if I am wrong let me know.
Again, I think I would check on local rental companies before I gave up on the idea. Although I am not too thrilled with renting any motorized vehicles on cruises, if you have you heart set on this, then look into it more.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013