Trip Advisor has a search function... below is a link to a list of threads referring to Whale Sharks. There is a relevance option, choose by date and all the threads from this year will appear at the top.
https://www.tripadvisor.com/SearchForums?ff=4482&geo=292019&scope=2&q=Whale+Sharks&pid=34633&s=DEdited: 06 July 2018, 11:52
you won't see whale sharks in the same quantities around Utila as you do at Isla Mujeres. However, they are around Utila all year, the deciding factor re. whether you see them is generally the weather and the luna cycle. We most often see them the week leading up to the full moon in March, April, May and September. The water has to be very calm on the northside of the island to spot them as they follow tuna schools feeding on bait balls at the surface. Unlike Isla Mujeres it is rare to see more 1-2 whale sharks at one time, although this year has been particularly good for also seeing Mobula Rays and Silky Sharks with them feeding on the bait fish.
I really don't understand the difference between conditions on Utila and on Roatan, but in my own experience I saw loads of whale sharks off the northeast coast of Utila in a single week in April a year ago and have never seen even one in hundreds of dive trips around Roatan. WE have seen frequent tuna boils around Roatan, the exact same conditions that attract them in Utila, but, alas, no biggies. Around Utila you'd see that ring of tuna jumping around, and very soon you'd see the whale shark tail break the surface. The boat would unload its snorkelers, and once I almost landed on top of one. On Roatan we have waited and waited and waited for something to show itself amid all the krill and tuna and nada. Otherwise the diving around Roatan was far better, so I can't really recommend that you skip Roatan in favor or Utila, unless you gotta work on your bucket list.
Our last visit in April was the best diving I've experienced to date off Roatan. On previous trips we visited in Feb and march and it was all about the micro, but this time was all about the macro.
On one dive alone we saw six turtles, green and hawksbill, 2 of the biggest king crabs you'll ever see, an eagle ray and we were in a fish-bowl ! It was just one of those dives that kept on delivering.
While we were there we heard reports of black tip and reef sharks being spotted swimming along the wall. On another dive a group of snorkelers who tagged along saw a whale shark - confirmed by the DM who was leading the group - but I did not :(. On our way out to a dive site we had three dolphins swim just ahead of the bow of the boat.
Just thinking about our last trip made me go back to my log book, here's what I reported from our last trip in April: golden chain moray eel, southern and eagle rays, green and hawksbill turtles, scorpion fish in addition to some fantastic mico species like pipefish, juvenile and adult drum, lined sole, and from what I can read, 'a squal of anemone...shrimp'..?
I got off topic here.
I get the sense there's something special about diving and even snorkelling in April...
If you really want to snorkel with the whale sharks go to Mexico(Cancun) from June thru August and most likely you will be able to swim with many. We went June 2nd and saw about 15. Had an awesome time. Whale sharks are hit and miss in the Bay Islands. If you want to dive with whale sharks, go to Thailand, Similan and Surin Islands. There is a lot to see off shore of Roatan, but have yet to see a whale shark!