San Carlos sits where Lake Nicaragua empties into the Río San Juan, and it is the junction for everything in the southeast: boats downriver to El Castillo and the Indio Maíz reserve, out to the Solentiname islands, across to Costa Rica at Los Chiles, and the long ferry back up the lake to Granada.
The town is a working port rather than an attraction — a fort ruin above the waterfront, a malecón, and a market — and most people arrive, sleep once and leave on the morning boat. The ferry from Granada takes around fourteen hours and is an experience in its own right.