There are wave runner rentals available on both islands. I would save the wave runners for Freeport as there isn't really too much to do there so it is a nice way to blow some time, i think Nassau has more worth seeing.
I think we rented them for around $80 per hour...you can try to negoitate a bit, if they aren't busy they may come down a little. There is also a jetski tour in Freeport you can look into....that takes you all around the island.