Knocking 10 enemies in to the water can be a tedious task. The further you progress in to the story, the fewer the opportunities to work towards this trophy there are. I suggest the Container Terminal 7 area, which has Dragnov as a final boss.
In this stage there are multiple enemies which are just hanging out by the ledge that can be pushed in. If you have Devil Jin, you can easily laser beam them one by one in to the water without alerting the others. if you have to run up this will draw their attention, however you still should net one or two per section of the map.
Weapons such as the gatling gun or even the crowbar can be helpful for knocking enemies out of the "ring" as well. It doesn't require excessive force, and you can even bounce enemies who have already been K.O'd in to the water for this trophy. You'll know you have successfully done this when you see the 2,000 appear in white numbers as their body falls in to the water.