The "Estonia" sank in 1994 in the stormy waters of the Baltic Sea. Only a few people survived this tragedy. Pärnu has a monument dedicated to the victims of the sinking. It is located near the harbour.
Budapest8 has a lot of interesting information about the disaster on his page.
To be honest I don't remember how to get there, since our host family drove there for us, and pick us up later down the river.
The water is very quiet, so no experience needed, you can even stop in the middle and swim if the fishermen don't get too angry ;-)
Good relaxing experience.