Be sure to head in knowing you're very unlikely to see whalesharks with just a few days to look. It could happen, but be sure you'll feel satisfied with spending so much of your extra time there even if you don't. Here's my favorite place to stay on Utila:

aboututila.com/AccomInfo/…Index.htm

Snorkeling from shore is available but you really have to look for it. If you can find the area near the Airport Caves dive sites, it's the best we've found. You have to walk across the bridge and down the road toward the old airport area; when the road curves to the left, go with it, then be looking for an opening on the right to cut between houses or through a lot with a partially built home. High tide makes it easier to get through the reef to the outside.

For what it's worth, I'd consider adding to your time in Copán - it's worth at least 3 or 4 nights imo. There's some wonderful rafting, kayaking, waterfalls, hiking, horses, itself just outside LaCeiba, too - another nice counterpoint to island time. We loved Omega Lodge:

www.omegatours.info

Have a great trip, whatever you decide!