Utila is arguably the cheapest place on earth to learn to dive, and a basic PADI course will cost around $150 (pounds 90), usually including accommodation. There are beaches but they are a long walk from the guest-houses and often plagued by mosquitoes, so Utila's raison d'etre is snorkelling and diving.
There are a large number of dive schools, which means there are classes starting every day, hence the low prices. The Lonely Planet Guide to Central America provides good advice on how to choose a dive shop - look for the safest not the cheapest. For the most up-to-date information it's best to talk to other users on arrival, but personally I recommend the Utila Dive Centre, which has experienced instructors and has been established since 1991. It also provides rooms in the Mango Inn, which although a long walk from the dive centre, has comfortable rooms and delicious food.
Most dive shops include accommodation or discounted rooms with their dive package. Seven nights at the Mango Inn (www.utiladive centre.com/html/body_our_hotel. html) range from $229 to $329 for a bungalow with air-con, including three dives a day. The Laguna Beach Resort (bookings via Los Angeles, fax: 00 1 318 893 5024) offers fully inclusive six-night dive packages for $699, including air-conditioned rooms, six days diving and all meals.
Iberia, Continental Airlines and American all do daily flights (from Miami) to San Pedro Sula in Honduras. From there flights to Utila leave twice a day.