It doesn't have to be a closed ring. Or resemble a ring.

Many metro systems in other big cities with comparable topological constraints have metro lines that are orthogonal to those going out in roughly a star pattern from the center.

But NYC almost only has the latter.

Underwater sections are not an issue, really. There are many cities that have metro lines going under bodies of water.

And even the length or depth required is not an issue if you want to build it.

That's why you can go from England to France, by train, in roughly half an hour, under the English Channel.