My understanding is if the strike was to target survivors that could be considered a war crime. If however the strike was to sink the boat and as a result those two survivors died it would not be a war crime. AFAIK there is no duty to make sure all survivors are out of harm’s way when a military action continues beyond the initial action. It would be different if the survivors had been picked up and summarily executed.
Where I think the left is going is stating that Hegseth, after the initial strike and realizing there were two survivors, gave an order “to kill them all”, and a second strike was targeted at the survivors.
I highly doubt Hegseth gave any such order. I am not well versed enough in laws which would apply in taking out suspected narco-terrorists to opine for sure it would be subject to rules of war.