Come on mate, it's not like you calmly walked up and picked that guy up off the rock or swam up and grabbed it while it swam toward you. It would have been underwater probably sitting on the bottom, thinking it is camouflaged, after seeing you enter the water and you swam down to get it. Either that or had to swim after it to catch it. They have the same fight or flight as any other animal. It is hardwired and not something that can be switched off unless exposed to a 'threat' for long enough to realise there is no threat.