Fun on the ship but shore excursions are hard to grasp

The days at sea are fun - lots of good food, music, shows, swimming, sunshine and sea view. Staff are attentive and helpful. My daughter (4) loves the kids camp room on the ship. My son (6) basically swam all the time and if he was not swimming, he had ice creams with his friends on the top deck. However, the day on Cuzumel, the island, just sucks. We went to the assigned deck on time but waited at least half an hour to get off the ship. Once on land, we found ourselves in a crowed small shopping mall. So many people just stood there waiting for excursion groups or basically were too hot to go anywhere or do anything else. We had registered a submarine trip on ship so we found the group, went on a boat, then a submarine, but hadn't seen any colorful fish or coral reef because they had been destroyed so badly, there were basically nothing left to be seen. It was a waste of money($99/adult, $49/child). After that we wanted to go to a beach but there wasn't any beach at walking distance. We asked a man who stood nearby where to find a beach for the kids, he said we had to take on another excursion activity which includes snorkeling, otherwise no way to go, and he started bargaining with us.  We gave up, just went back to ship. Later we got to know that you could take a 10~15 minutes taxi to a nearby beach park, and to find a taxi, you should just walk out of the mall area. That man just lied to us! Can you believe it? Because of him, we went on a cruise vacation but didn't step on a beach, that is just ridiculous! Of course, it's basically our own fault. We should do more research on shore excursions before we go. And that's my advice to you, if you plan to go. Thanks!