We went in small boats with a guide that spoke both English and Spanish. At one point we zoomed thorugh a mangrove swamp to a boardwalk and got out to walk around. The boardwalk wasn't in good condition, but the walk was worth it. Lots of trees, some birds, and interesting reflections in the surrounding water.
They said we could probably go swimming there. Swimming was out of the question because of slight problem due to a local resident: a crocodile!
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