'Piranha 3D' Star -- My BF Could Beat Up Your BF

British model Kelly Brook -- who was eaten by killer fish in "Piranha 3D" -- dared to go back in the water in South Beach this week ... along with her wrecking machine of a boyfriend.

The jacked dude on Kelly's arm is David McIntosh -- a model/actor ... who formerly served in the British military as a Royal Marines Commando.

In fact, David says before he got into professional posing ... he served in Iraq, Afghanistan, Somalia and Northern Ireland.