On the ferry - $60 (1400 Lempira) R/T but it will be a
short day on Utila - the direct ferry goes direct Fri-Sun only at 10:45 and back at 3PM.
Home - Utila Ferry It must be the better part of an hour ride also.
Probably all you'd really have time to do is walk around town/go to the beach. West Bay on Roatan is a lot nicer beach. If you wanted to stay over there's a lot of cheap walk-up rooms/hotels available in town. aboututila.com lists most.
Afaik CM Airlines or Island Air only fly between Roatan and Utila on Saturday to meet the connecting flights to the States.
Roatan flights seem to revolve around the dive resorts Sat-Sat schedules to a large extent.
Check with Alice at Worldwide Travel on Utila - she may have other flight options.
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If you're trying to see the Whalesharks - yeah maybe - but that's kind of late. Optimal months are April/May typically but Feb-June is possible. IMO they've mostly left for summer off Mexico by mid-June. Also in my experience the likely place to see them was off the north side (deeper water) so that's another 30min. boat ride. Often those seem to run late morning/early afternoon - we did two morning dives b4 going to look for them and some of the snorkel tour boats were out at the same time.
The ferry pier is right downtown so just a few minutes walk were you to find something that ended by 2pm or so.
I did a WS Research week on Utila a few years ago - our optimum week was the first week of April. We snorkeled with 8. 3-4 days we didn't see any.