Near the U.S. the only place to dive with Whale Sharks is Belize in the late spring. Honduras, Mexico etc. are snorkel only.
The exception is the Socorros liveaboard trips - certain times of the year they encounter them there. The recent video of the diver cutting the WS free that some company is using as a TV ad now was filmed last year by Solmar V off Socorro.
It's been my experience (off Utila) that most of the time they're likely to be either indifferent or try to get away when they encounter snorkelers. We had one that let us watch him feed for a few seconds but most of them started diving deep as soon as we approached. One circled under the boat and made a second pass so I've got about 30secs. of that on video.
My buddy did the Belize dives - he's a photographer and had time to get some good shots since the WS's are attracted by the Cubera Snapper spawn and seemed indifferent to divers hovering around their meal source. No flash allowed. He said over 2 days of diving (4 dives) they probably had about 10 mins. of in-water close encounter. If you're interested, Splash Dive in Placencia is one option.
Overseas, Maldives, Philippines, Ningaloo, Australia or Tanzania are options. Maybe Yap/Palau also - seems like I recently read a trip report here that mentioned it - along with the Mantas. Ningaloo may be snorkel only also - I've got footage shot by a diver but it's shallow so he could've been snorkeling.