The standard route is from San Jorge, the small port a few kilometres outside the mainland city of Rivas. Ferries and lanchas (smaller boats) run every hour or two between San Jorge and Moyogalpa from early morning until late afternoon. The crossing takes about an hour on a ferry, slightly less on a lancha. Tickets are cheap and bought at the dock — there is no need to book ahead unless you are bringing a vehicle.
A second, slower ferry runs from Granada to Altagracia twice a week (currently Mondays and Thursdays, but check locally). The crossing takes around five hours and gives you a long, atmospheric voyage down Lake Nicaragua, but it is not always reliable.
To reach San Jorge from Managua, take a bus to Rivas (around three hours) and a short taxi or local bus to the port. From the south, buses run from the Costa Rica border at Peñas Blancas to Rivas.
Cars can be brought across on the ferries, but parking on the mainland is easy and a hire car is rarely worth the cost — local transport and bike or scooter rental on the island is cheaper and easier.