To cannonball into a body of water, use
to jump forward and then hold either
or
to curl up. The requirement for this trophy lies in being able to perform a cannonball by jumping from a high-enough spot without reaching the boundary where it will respawn you.
While navigating through the ruins and tunnels of Chapter 2, you might come across a stone walkway over a ruined ship alcove. You can jump from here, holding on to either of the two buttons mentioned, to cannonball into the water and receive your trophy.
* If there are any suggestions for an earlier location, or for an easier attempt, let me know and I will change my solution to reflect an alternative with credit.