It's illegal to shoot at soldiers who have surrendered in an conventional war. Then too taking POWs is a risky thing all it takes is one guy to cause a mayhem.Be practical. They are stranded in water after their boat sunk. It's illegal to shoot at them on suspicion of them carrying a weapon while they are about to drown. And there's no way they can swim to any shore from there, nor do they pose a risk to anyone from their position.
Jihadis and militants have no human rights, no court will bother taking your line of argument.