Bombing a school in an air war is slightly different from hitting a boat with a missile and then after its immobilized, murdering all the survivors.
One can be blamed on fog of war, the other is straight up murder, and all involved need to be prosecuted as such.
I'll admit it's hard for me to debate which war crime is worse. I see your point, but I just can't fathom scores of dead children, even if it's negligence rather than purposeful.