Maybe I was just lucky but my husband and I snorkeled with about 50 whale sharks last June (June 22nd in fact, which is my birthday) off of Isla Mujeres. It was the best 40th birthday present I could have asked for and one of the highlights of my life. I think May is a little early to see the whale sharks in Mexico, though.
As the others have said, from your criteria, I would go to Mexico over the other two places. We are researching Roatan now for this June and from what the other posters have said and from the research I've done, it does seem pretty rustic. We were thinking Belize also but it seems like you have to go really far out for good diving and the drive from the airport to the places you want to stay seems really far too.
As far as the bugs go, you're going to get them in all of those places. We were bitten alive everywhere we went in Mexico, including the upscale resort we stayed at. Just bring tons of bugspray.
Have you thought about staying on Isla Mujeres? There is a small, upscale boutique hotel called Villa Rolandi that I've heard great things about. Also La Amada north of Cancun is supposedly really nice and it is far away from the touristy area. We wound up staying at an all-inclusive which we hated and it doesn't seem like you guys would like that either. Or maybe look at VRBO for a vacation rental? We are thinking of doing this in Roatan.
As far as diving, both my husband and I hadn't dove in 10 years either but we found an AWESOME dive shop called Scorpio Divers that basically gave us a refresher course on our first dive with them. We wound up diving three days with them, it was so much fun. They picked us up from our hotel too, which was really convenient. We really consider them our friends now and are considering going back to Mexico in June just to dive with them. They also arranged the whale shark dive too.
I am always cold too. At that time of year the water was super warm. We dove in bathing suits most of the time, except when we did a deep wreck dive and a night dive. It was like bathwater!
We felt very safe in Mexico too. We rented a car one day and drove to Tulum and the Coba ruins and had no issues.
One more thing about the whale sharks. The boat ride out to see them was about an hour and the water was really choppy. Many of the people on the boat got seasick on the way out. Also, you are swimming in the open ocean so that may be a little freaky for some people. But you don't need any diving experience to snorkel with them as you have to wear either a life vest they give you or a wet suit if you have one. You need to be in pretty good shape to keep up with them though.
Good luck!
