I agree with what's been said above. In addition, rental rates are high because they are mostly catering to the 1-2 hour rentals for a ride around the island. If you bring your own, you won't be rushed and can ride whenever you want. As long as you have a decent lock, there are so many other bikes for thieves to choose from, you will be fine. We never had any problem with the handling on the ferries either. Have fun!