If you just want to stick your face in the water and look at the fishies then stop by any major sporting goods chain store and pick up the items suggested on your cruise ship to-do list. If you want to do serious diving, you need to go to Stingray.
Pablo, the owner is an avid diver, as are his employees and they'll help you find everything you need for you underwater adventure or answer questions you have about equipment.
Not yet initiated into the wonders of scuba? Pablo organizes classes in the store (pool time is local, your certification dive is a quick weekend away) with PADI certified dive masters.
Since the store does cater to serious divers it can be a little intimidating to a newbie. Do a little research so you know vaguely what's in store, but don't be afraid to ask questions.