In Cancun, we saw gulls, terns, herons, and other wading birds near the beach and other bodies of water, such as the lake near Coba. Pelicans flew over the waves, dropping in to catch a fish, and taking off from a rising wave.
One night we saw frigate birds circling above the beach against the darkened sky. These birds have a white patch on their neck, and we took quite a while figuring out that the patch was reflecting the moonlight, making the birds very visible in a really spooky way. We saw frigate birds soaring all the time when we were on the hotel beach and along the shore on Isla Mujeres. In the photo they are soaring above me: you may need to click on the image and see a larger version to get a look at them.
The strong winds that blew all week reduced our opportunities for fish watching, though one morning we went in despite a red flag on the Club Med beach adjacent to our hotel beach. We snorkeled around several large coral formations, and saw a large sting ray swimming just under us, we approached a barracuda around 5-6 feet long (and then backed off), and we also saw very big angel fish and parrot fish. As we returned to shore, the life guard approached. He asked us about the visibility and conditions. We said everything was ok. He changed the flag from red to yellow, and soon afterwards several other groups of snorkelers were in the water.