So where is the Army supposed to defend?
First, blowing up boats is not a defensive act in my opinion. If Coast Guard had tried to board them according to our law and were fired up, light them up.
Second, I'm not comfortable with the military being use to enforce Congressional law. That is a Law Enforcement role.
Third, is your argument that that US military has the right to assassinate any individual, of any nationality, located in any other country or international waters, based on the idea that they MIGHT eventually cross our border illegally...someday?
Blowing up a boat in international waters is generally illegal and violates international law, with narrow exceptions for self-defense during an active armed conflict.
Were these boats firing on US forces? Had they fired on Coast Guard?