This excursion was very fun for DW and I in May.
To get a feel for the length of the ride, Gordo Bank (aka the sandbar) can be seen with Castaway Cay from the satellite photos on Google Maps. The sandbar is shaped a little like the number 3 and is east of CC.
http://maps.google.com/maps?ll=26.081543,-77.522278&spn=0.039740,0.062399&t=k&hl=en

http://maps.google.com/maps?ll=26.081543,-77.522278&spn=0.039740,0.062399&t=k&hl=en