If the people of Gaza are "willing to provide a sea in which Hamas can swim", then the people of Gaza ARE Hamas. We are currently distinguishing between Hamas' fighting branch and support branch, with the "civilian" support branch being considered an illegal target.
That's not how we fought WW2, the last war we actually won. Then, the entire population of the enemy side was considered "fair game". We bombed the cities of Germany and Japan, knowingly killing hundreds of thousands of civilians. And that's what finally broke them.
The war will not end until the wives and daughters of the enemy beg their men to make it stop.
Did not break Germany, and didn't break Japan till we graduated to nukes.
The terror bombing of cities, especially in Germany, killed hundreds of thousands of civilians, but had little impact on the German war effort, at least relative to our resources used. It arguably was a war crime, at least in modern terms, and certainly by those of the 19th century.
Are you willing to have this criteria applied to America? If so, terrorist attacks here by an enemy with which we are at war would be legitimate, at least if the enemy donned uniforms before attacking.