I live in San Pedro and have taken this route 9 times. It works out great if you make a mini vacation out of it and hang out in Playa Del Carmen, Tulum, Akumal, etc for a few days first. Otherwise it is a lot of travel. Regarding the water taxi from Chetumal to San Pedro. Be SURE that the cab driver you get at the ADO station knows exactly where you want to go and that is the Muelle Fiscal, to pick up the water taxi. Last time I went through, it was a cluster%&$* as the cab driver brought me to the Corozal border, not having any idea about the water taxi and thinking I was just a dumb tourist and not caring at all about where I was actually going. Then charged me 200 pesos for the ride. Crossing the border from Chetumal to Corozal is also a cluster%&$* and the water taxi from Corozal to San Pedro only runs 1x a day, so you would then need to take a flight, or spend another night in Corozal. Other than that, flying to Cancun and taking the ADO bus is a breeze.