This lagoon (it shares the same ecosystem as Los Haitises south of Península de Samaná) at the northern end of Calle Duarte was once Río San Juan’s claim to fame. Unfortunately, overuse and pollution mean the lagoon is no longer pristine and swimming not recommended. It’s still fairly picturesque and a dozen or more boatmen offer hour-long tours (US$35 for up to seven people) through tangled mangrove channels, including interesting rock formations and a cave populated by hundreds of swallows.
Look for a small shack next to the public bathrooms down by the Laguna – you’ll find it easier to join a group on weekends, when Dominicans come to take this trip.
You can also visit the lagoon on foot – there’s a path on the far side of the Hotel Bahía Blanca along the water’s edge into the mangroves.