There is an estuary on the west side of Tamarindo that runs a mile or two inland. You can take a ride with a local boatman who points out the many form of wildlife. There are lots of species of birds and sneaky predators like this crocodile.
No running your hand through the water from the boat. However, I have seen boys swimming around the area where the boats are moored. Perhaps as a result, the sign was recently installed.