The sweet spot is between late November and April. This is the dry season with comfortable temperatures, fewer showers, and less humidity. Mid-week and early morning arrivals catch more space on prime beaches. The summer months (June-August) bring warmer sea temperatures and more rain, but also fewer crowds.
If you’re taking a day trip to Isla Mujeres, aim for a morning departure so you get some time on the island before the crowds build. This is also the best time to catch the sunset on your ride back to Cancun at the end of your tour.