The only constitutionaly legal action the American state might invoke against Americans on Gaza bound vessels would be indictments for treason; and that could be only AFTER Congress enacts a Declaration of War against Gaza. So what's the difference between an American mercenary in Afghanistan and an American citizen aboard the Tahrir? Absolutely nada!
First U.S.-trained Syria rebel believed killed in fighting, U.S. officials say (Reuters)
from the article: Israel Navy intercepts Gaza-bound aid vessels; no injuries reported