20-Year Prospect
Take a look at a globe. Notice anything...blue? Yeah. There's a lot of that color. And as long as the majority of our planet is covered in water, you can bet we're going to want to build stuff in, on, or around it.
There will inevitably be new underwater construction, but the need for the repair and maintenance of current construction is only going to grow. Maybe this is why the field of commercial diving is expected to grow at a faster than average rate; the more stuff we put down under, the more people we need to make sure it stays intact down there (source).
That said, commercial diving is a young person's game. With what's essentially an age cap for people over forty-five, your personal twenty-year prospect is going to really depend on when you get started. Eighteen? Tubular. Forty-four? Eh, maybe a little too late to the party.