Never have rented a Harley while on one of the islands. I have thought about it a couple of times, but since I usually have my family with me, its not practical.
If this is something you want to do, I say go for it. But be very careful as driving a bike there is more dangerous than doing so in the States...
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
NCL Dream October 2006, RCCL Mariner OTS June 2007