Oh, you guys are making it so difficult, I had chosen to just stick with Roatan for my first time in the Bay Islands, I figure string of pearls, diving with sharks, etc would be plenty and save what limited time I have and skipping the extra jaunt to Utila but this talk of whale sharks is tempting......so so so tempting.
The big question is WHEN are you going? Peak whaleshark season is either early spring or (from what I've recently read) in September. By now most of the sharks will likely have moved to Gladden Spit for the snapper spawn - after that they appear to move to the Los Mujeres area for the summer. But no one really knows for sure.
You seem budget and time constrained. It's going to cost you a minimum of $100 (r/t ferry rides) to get to Utila from Roatan. And you'll burn 1/2 day each way doing it. Or you can fly over and back for more. Utila has a small, unlit airport so it's day flights only.
All the ones we saw - 6-8 (maybe the same one thrice) over two days - were off the north side so that requires a paid boat ride. We spent approximately 2-3 hrs. each day on the boat looking for them.
Notice that laurenceh dove with them
last month. That's the "season". Not that they aren't spotted all year - they are - but it's much more infrequent. Steve Fox from Deep Blue told me he'd seen one every month for a year. He's also an expert on them, runs a program to document them and lives there.
I think segmenting a week long trip for the chance is just going to cost you money and buy you some good diving off Utila. Really what's the likelihood of one being spotted during the several days you're there - in the same area. The boat captains talk over the VHF to others nearby when they find one - but you have to be out there actively looking. Most operators do morning boat dives then do that.
Even if you do find one, most of them dive deep when the snorkelers hit the water. You maybe get a second look when they surface 10-15mins. later. Also there's a group of boats often hovering nearby - there's a protocol for dropping divers on one. It's also limited to a number of snorkelers per boat - but from what we saw that wasn't always enforced. When 20-30 people hit the water simultaneously - you have about 2 seconds before they're gone.
In the very heart of peak whale shark season several years ago we spent an entire week there. We saw all of ours the 1st two days then it was flat calm and hot which drove them deep - we never saw another one. Granted the ones we saw/snorkeled with made it worthwhile. However 2 of them were just tails going deep by the time we jumped also.
ymmv, if you really want to have the best shot - I'd suggest going next year - during the equivalent 2014weeks (they'll be different) shown here:
Deep Blue Utila - swim with whale sharks at our all inclusive resort. Utila, Bay Islands, Honduras