To be clear, the time back at the resort on the SI is only 5-10 minutes....enough to pee at the near-dock toilet but not to change clothes or get lunch.
The reefs were fine, for Cozumel. Sponge-dominated not lush with corals. Go elsewhere for fields of healthy hard and soft corals. Good fish life, but not that diverse. A few turtles, rays, sharks. No trumpetfush.