I've done lots of dives while on cruises. My recommendation is to book through the ship. It may be a bit more expensive, but you don't need to worry about the ship arriving too late to go on the dive or leaving without you. Also, in Belize, a dive booked through the ship will pick you up at the ship whereas a dive booked elsewhere will require you to tender to shore first.
BTW: The GP is a great ship (except that the midship stairway does not go all the way up).
Also: There are a number of threads on cruising and diving with some great tips (like dive knives may be confiscated by security). Also, beverage cups in the buffet are too small -- bring your own plastic cup or plan on lots of refils of their 4 oz cups!