https://en.wikipedia.org/wiki/Polder

Type 1 is often an island situated below sea level.

For instance https://en.wikipedia.org/wiki/Flevopolder . Island. Surrounded by open water because that's actually a good idea. Below sea level. 400 000 inhabitants. 2 cities, major agriculture, minor airport.

Ever wanted to grab dinner on the sea floor? Visit Almere Center. Though lots of people find it to be a bit boring in person.

Want the same sort of thing in the US? Consider dropping the Jones act. Right now it's illegal to bring the equipment that builds these things into the US.