The easiest way to get to Ometepe is to catch a boat from San Jorge, a small town just outside of Rivas. The journey takes about an hour, and there are ferry departures at 7:45 a.m., 10:30 a.m., 2:30 p.m. and 5:30 p.m. Ferries return to the mainland from Moyogalpa at 6 a.m., 9 a.m., 12:30 p.m. and 4:30 p.m. The trip should cost $1.60.
There are also smaller boats that ply the same route, with far more frequent departures. These leave from the same spot, and cost $2.10 per person. Be warned however that the trip can be a rough one. If you're prone to seasickness, you're better off sticking with the larger, more stable ferries.
There is also a ferry that leaves Granada for Altagracia on Monday and Thursday afternoons. The trip takes around three hours and is notoriously unsteady. If you really love boats, you can hop on at Altagracia for the 14-hour trip to San Carlos, near the Costa Rican border. You'll be much more comfortable if you bring a hammock, as well as some seasickness pills. It's not a fun trip, and it’s not recommended unless you want to head up the river into the Rio San Juan region and Nicaragua's rugged southeast. If you're planning on crossing the border directly into Costa Rica it’s advisable to do it by bus instead – it will be cheaper, faster and a damn bit more comfortable. Crossing from San Carlos requires waiting around for a boat (just what you need after 14 hours on a boat) and then probably spending the rest of the day waiting for a bus down to San Jose.