Since i played this game online on the 360, i can't tell how big the difference on the other consoles will be.
After i finished the game, i loaded the 2nd Chapter on Easy Mode. (Didn't have much luck with the 3rd Chapter but this may be a matter of taste)
I gently tapped
on the first guy with the Sonic Ammo, and kept shooting him until he was next to a glass door.
After i shot him dead (because of the key), i opened the door and kept tapping and running away from the 2 other guys who were hiding in a corner, until they were next to multiple glass doors.
Apparently you don't have to kill them with the water, because they were alive, even after the drowning/bubbles symbol.
Hopefully this will help you out.