Sub booking questions

[Honeyroastedmelnut]

When I'm in Grand Cayman, I would really like to do the underwater sub. I've checked out their website and it is almost half price of the cruise excursion (no real shocker there), my question is should I really book in advance or can I get my ticket the day I'm in port.

Carnival has already changed the order of my itenerary once since I booked the cruise and with this tender-thing going on in Belize I won't be surprised if it changes again. I would prefer to not have to keep up with the order of port of call changes and have to keep changing bookings with the venders.

Also, I plan on going to the Turtle Farm (again not through Carnival but taking the public transportation). I would like the freedom of doing the sub and turtle farm at my own pace and not have to worry about booking a time that would make me feel rushed at the turtle farm or worried about getting back to the boat on time.