safety is a non issue. the issue with May is the sargassum seaweed can get pretty bad depending where you go. we went in may 2013 to secrets maroma and royal hideaway and in may 2023 to Andaz mayakoba and Hyatt zilara Maya riviera and the seaweed was much worse the second time we went as it's gotten a lot worse over the years. Its peak is may June iirc. We went to vieques Puerto Rico in the spring 6 or 7 years ago and the sargassum made the beaches unbearable for us there, it can really ruin things if you're main focus is the beaches
Somewhere like mayakoba is nice because its a little more laid back and they have a barrier off shore that protects the shore from the seaweed and mexicans picking up what does make it to shore where as Hyatt zilara didn't have that like mayakoba and it ruined the beach. definitely recommend adults only and The Hyatt ziva/zilara Cancun (they're next to each other) if you did stay cancun might be worth a look as its at the tip of Cancun so you have two beach options facing different directions so you theoretically should have one side that's seaweed free. ocean tours Isla contoy Isla mujeres was a pretty fun adventure or the Tulum ruins/ cenote swim tours are also a fun time, look up ratings on trip advisor to find the reputable ones.
here you can see how the barrier protects mayakoba and makes the beaches still very nice for swimming. Andaz is getting renovated to Alila there and there's also a Fairmont and some other resorts in the complex.
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Isla contoy
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