WikiVoyage states that you can get to Isla Juventud (Nueva Gerona) by plane or by boat:
The boat ("Kometa" catamaran) costs 50 pesos per person (25 for children of less than 12 years of age):
50 CUC for foreigners
50 CUP for locals.
I've checked with Cubana and the price to go by plane is around 40 CUC per leg. The Lonely Planet (2007) explains that there are other ships, not just the Kometa.
There is a bit weird to pay more for a 2 hours road trip plus a 3 hours ship trip than for a comfortable 30 minute flight... Is this information right? Which is the cheapest way to get to Isla de la Juventud for a non-local?