Another thing to consider is what kind of jack and lug wrench you have on your tow vehicle. You can carry a spare tire on your boat trailer all day long but if you have no means of jacking it up and/or removing the lug nuts, you're outta luck. I picked up a bottle jack at harbor freight, both inexpensive and easy to store, and a collapsible lug wrench that came with 4 socket options. You can easily store them on the boat to always have them or store them on a "dedicated" tow vehicle.

http://www.amazon.com/Dorman-711-900...rds=lug+wrench