Hi booth22,
Dollars are accepted most everywhere on Utila, although as mentioned it may be beneficial to trade to local currency before shopping. Several of the stores, especially groceries etc, have their exchange rate posted right outside or at the counter, so just check before you shop. Pretty much standard right now is 18.95 - 19.0 to the $1.
Electric is the same cycles as the US, so no converter is necessary.... but... if you'll be charging cameras, laptops or other electronic devices, you may want to bring a small surge protector. I've found them helpful whenever traveling to an area where power is supplied by generators. Better safe then sorry, cause it ain't easy to find replacements on an island.
Hope the info helps. Enjoy your stay in Utila, its a fantastic little jewel of an island!