This area in Boston Common is home to a named super mutant behemoth named Swan who is staying still in the middle of the pond. He will only attack you if you attack him first or if you (or your follower) gets too near him. Killing Swan will allow you to loot the Furious Power Fist from his body.
To the NW of the pond is a gazebo with several highly radioactive barrels. Beside the skeleton is an issue of Crognak the Barbarian. Pop a rad-x then pick it up immediately.
The swam boat has a Groundkeeper's Log holotape in the middle seat. There's a green steamer trunk inside his shack, as well as some notes on the mattress and on the crate.