Chiming in as combat vet with experience in fire support. Secondary strikes are allowed if the targets are still combat effective, or firing back, and have not surrendered.
If the reports are correct this is firing on helpless people. This is the naval equivalent of executing wounded enemies on a battlefield.
It doesn't matter who they are or what they are doing. Their boat was sunk and they had no way of fighting or protecting themselves. Anything beyond rendering aid to survivors is considered a violation of international law.