Are you sure its just the oceans and not the massive navy we have in them?

Aircraft carriers are basically by definition not defensive.

A navy is an effective mobile border when you don't have a Hawaii everywhere. It's easier to defend your castle when your first wall is 2000 miles from your natural border.

Plus the best defense is a good offense. Nukes aren't a defensive weapon and yet they're an effective deterrent.

Yes. Invasion is basically logistically impossible without the assistance of Canada or Mexico, and with their assistance, would require a large, obvious, and vulnerable preparation lasting at least many months.

Of course our having a navy is a good idea. I'm obviously (and I think you know this) not advocating complete disarmament. We probably don't need a navy large enough to project force to the literal other side of the world in multiple places at the same time, though. Nor a standing army large enough to invade and occupy 2+ weaker countries at the same time.

The vast majority of our military spending is about hegemony maintenance and the ability to impose our will around the world, not defending ourselves. Our oceans do in fact put us on easy mode, for simply defending ourselves—we don't need the world's strongest military to do it.