WMG / Secret Wars II

Namor knows where the government sank that gold skyscraper.

It seems likely he would know, and if that's true, he could access it easily. That said, he probably doesn't have much reason to. It's been established since his appearances in the early FF comics that because he has easy access to countless shipwrecks, Namor has enormous wealth on the rare occasions has actually needs to spend money on the surface. To him, the gold skyscraper would probably seem superfluous. But it must be nice to know it's there if he needs it.