Well, we never see Silvia after she falls from the boat, so presumably she's dead.
As far as whether or not Barbara and Irem are thieves who kill their victims, it's hard to say. On the one hand, Giorgio seems to be have been a genuinely dangerous figure who probably would have tried to kill them. Hence, their actions could be considered self defense, though the way they go about it is rather bizarre.
But then their behaviour at the very end, seems a bit bizarre. Perhaps they were even more disturbed than Giorgio? Maybe Irem really did rig Giorgio's regulator so it would stop working in the middle of a dive? Who knows?