The problem is range. What if the target is far from the water?
True, naval guns would be of little use in Afghanistan, and 16 inch (or bigger) guns are really too heavy to transport over land. So you have a valid point. But when one considers modern ballistics can increase the range of big guns to 50-100 miles vrs 25 miles for a WW2 gun alot of targets fall within the range of a modern main naval battle gun should anyone ever decide to build one. Space born Kinetic energy weapons would work well for this purpose too. A bit more expensive but a stream of cannon balls with a V of 20,000 mph would do a lot of damage to a harden bunker.