If you want to absolutely nail a chance to swim with whale sharks, look into doing a Isla Mujeres, Mexico whale shark tours. We've gone twice. Loved everything about the trip. The hotel we stayed at is kind of basic, but clean and safe. The hours in the water...spectacular. We felt safe there...the boat operators know what they are doing and the whale sharks were treated well...and there were hundreds of them...along with big mantas, mobula rays and lots of other fun ocean live. Highly recommended. By the way...we did Holbox island as well...loved it but the boat rides are often long and rough at Holbox.