It’s quite easy to travel to Nicaragua and there are no visa requirements for citizens of most countries. A quick search of any airfare site (Kayak, Expedia, Orbitz) will offer many affordable flights to Managua (MGA), the nearest (and only) international airport. From the airport, Granada is an hour away by taxi or other hired transportation service (usually $35 for the first passenger and $10 for each additional).
The airports in the US from which one can fly non-stop to Managua are Houston (3.5 hours), Miami (2.5 hours), and Ft. Lauderdale (2.5 hours).
Direct flights are also available from several Latin American international airports including Mexico City, Panama City, San Jose (CR), San Salvador, and Havana.
The major air carriers serving MGA include United, TACA, Copa, Delta, Spirit, Aeromexico, and American.
To get to El Espino from Granada (see map) one need only hire a taxi for the short ride to the port then a water taxi to the island (total time is about 30 minutes). The total distance from the Parque Central is about 4.5 miles (just over 7 km) with the last 1.25 miles requiring a boat.