The Indian River flows from the hills behind Portsmouth into the sea at the town's edge, and a short stretch of it — roughly two kilometres — is navigable only by small rowboat through an increasingly dense tunnel of bwa mang mangroves. The guides who work this stretch are government-licensed, and the trip is one of the most atmospheric things to do on the island.
The boats are rowed, not motorised, so the only sounds are paddles and water. The canopy closes overhead as you move inland, and the trunks of the mangroves arch into the water on both sides. You're likely to see herons, iguanas, and various river birds, and possibly a boa constrictor in the branches if you're lucky. The trip includes a stop at a small bush bar in the forest. Hire a guide at the dock near the river mouth; the going rate is negotiable but expect around EC$60–80 per person.