1. Get a tent in your hands
2. When start swimming, the tent will be dropped on the ground
3. Pick it up while swimming
4. Choose "pitch tent" anywhere

You can enter the tent by swimming into the entrance, your character will stand up. The unpacked tent will end up on the water surface, being recoverable.
Not tested if it works from a backpack. Tested however if it works in the ocean, the game prevents you from that.

http://imgur.com/a/sgwVl