Tarawa. After that, the Marines put in a swimming requirement. Apparently it's not that tough to pass the first level, but you do have to be able to swim in uniform with a rubber rifle.
They have Level 3 as a requirement, then you can choose to try L 2, and then L 1, which is in fact tough. My daugter said she was the first female to pass it in several years. My son was the only one in his platoon to attempt it.
The Army of course effected several amphibious assaults in Europe in WW II with zero Marine help, and they tended to be massive multidivisional efforts.
Higgins boats. They probably could have used some Hueys in some of those. I read about three navy destroyers risking grounding to provide fire support up close at Omaha.