20-Year Prospect
Rivers always have and always will dump sediment as they flow through ports. That's the bread and butter of a dredger's trade, and it's not likely to change in the next twenty years. Hundreds of ports and thousands of miles of waterways need to be dredged regularly, many on an annual basis (source).
In addition to that near-constant cycle, there are increasingly large container ships that need deeper waterways in the ports. That means more digging of even deeper ports, until one day you break through the earth's mantle and destroy the city. Okay, maybe that's not likely to happen—or at least not in your lifetime.
One of the biggest challenges you'll face in getting into the business is career stability. Dredgers tend to stay put their whole careers, which really puts a damper on job openings.
And it's not like other industries that continually expand. The U.S. Department of Labor only anticipates a few dozen new jobs created in the field each year (source). In fact, new dredging ships are trending toward modern designs that require fewer crew members to run them.
Sure, someday someone will figure out how to teleport physical matter across the ocean, or develop a ray gun that restores coastlines and builds up islands and berms. But the U.S. Department of Labor doesn't project any activity in those areas in the next twenty years, so dredging looks to be a stable and consistent need for decades to come.