This is an intended Arma 3 mechanic and is completely counter-able - for one, I can only stay under for so long and have to go up for a while the longer I stay under giving you an opportunity to kill me. For two, you can use weaponry specifically designed to be used under water such as the SDAR...
Perhaps if you were so upset about this at the time you could have messaged me and I could have maybe had a recording as you can struggle to hear a lot of what is said, which I'll get onto. To add onto that, you pop up and come to us after I've already been chatting with your lot so I find it...