I doubt there is a direct flight ORD-RTB. You can however connect through Atlanta on Delta and be there by about 1PM. Or Continental through Houston.
It would be a fluke if you saw Whalesharks off Roatan. Contrary to what some websites lead you to believe. I asked an instructor about it when we were there last year and (after he stopped laughing) his advice was go to Utila. Another told me he saw one once off Spooky Channel - he's been there
7 years.
Whale Shark season on Utila is Feb-May. Although they have been spotted every month this year. We went to Utila the last week of March. Saw/
Snorkeled with 6 Whalesharks on two different days. You can't
dive with them unless you're a certified researcher. And they don't typically swim in dive depth waters, after our dives we headed out into deep water to find them. They appear to like to feed at depth, every one we saw was in deep water.
fwiw, you get about 10-15 seconds in the water with them before they either dive or swim away - a
lot faster than you can.
We stayed at
Deep Blue Utila, they're part of the
Whale Shark Research Institute. We were there during one of the Research weeks, in addition to lectures and other activities, they make more of an effort to look for them during surface intervals. And use a spotter boat those 4 weeks. Just about every time we found one there were 2 or 3 other boats from different diveops nearby - once anyone finds one, they talk about it on the VHF.
Having done both Roatan and Utila now, I thought the diving was better off Utila also. Certainly a lot more interesting in the first 60'.