They were serious, professional men; Travolta’s character gave infrangible orders; and one of the killers was the deceased’s brothers.
Why wouldn’t they shoot Castle in the head? Why wouldn’t they check his pulse after a chest shot? Why not just be 100% sure he was dead?
Probably hit and run. Not want to cause a lot of local buzz with all the gunshots going off and the explosion on the dock by sticking around and getting noticed in broad daylight. Could have taken awhile to recover Castle's body from the water. It's implied that it's a miracle Frank survived the ordeal, in no small part out of his sheer lust for retaliation.
~~/o/