There are separate beginner and advanced dives and dive sites. The discover scuba class was worth it if you have not dived before because it lets you see if diving is for you. We did the class on the Sunday of our cruise (we did not really want to get off the ship anyway) and we spent about an hour in the show lounge going over basics. The instructors were very friendly and easy going. The "test" was pretty simple. Nobody failed...go figure. Then we went up to the pool deck and suited up. We spent the entire 1 hour under water going over basic movements and hand signals. Although you are not deep it gives a good sense of how you react to breathing under water and getting used the equipment.
We were then eligible for the 2 basic dives that were offered the rest of the week.