PhotoBret is right you can see a lot around the docks right here - in the last week notable snorkel sightings under and around the dock have been an eagle ray, sea horse, bat fish, green moray (he hangs out here).
Close by is the reef we call the airport reef... It has an entry point about 10 minutes walk from here. In calm weather it has some of the best snorkeling around the island due to the reef tabletop being very shallow there (around 5 to 15 feet) and the drop off having lots of interesting features to look at and a good corals and plenty of marine life. In rougher weather we recommend heading to an area inside the bay called Blue Bayou. Easier access to the water than the airport reef but not (imo -but others disagree with me) so good snorkeling. Reef is shallow again, and you can see plenty - not uncommon to sight hawksbill turtle along this stretch of reef. A bit of a walk or a $1.5 cab ride.
In general snorkeling from the boats is good with the vast majority of Utila's dive sites great for snorkelers as well as divers