For a poor state that can't afford the large military it has while trying to keep their internal security people happy enough to keep the ruling elite in power and while trying to develop nuclear weapons, building up their navy seems to be an odd allocation of scarce resources.
Really, focusing on their air force would be the only thing North Korea could spend their money on that would be more futile than trying to challenge America, Japan, and South Korea at sea:
North Korea is dumping “serious resources” into its naval forces, says a U.S.-based think-tank.
Satellite images show North Korea has been making “aggressive efforts” to upgrade its primary naval base, reports the U.S.-Korea Institute’s “38 North” blog. The Munchon Naval Base at Wonsan, which sits beside the Sea of Japan, is North Korea’s largest naval base. It’s a major focus of North Korean military spending. North Korea aims to improve the training facilities, conventional weapons systems, special operations capabilities and new ship support facilities.
The article mentions that the entire North Korean navy sniper brigade will be based there as well as hovercraft and missile boats.
The article says it will give North Korea better coastal defenses and better ability to launch an amphibious attack on South Korea.
Neither of those objectives make much sense.
If North Korea is to stand a chance of winning a conventional war, an east coast amphibious capability is pointless when the only thing that could possibly shake South Korea enough for North Korea to win a war is the fall of Seoul in the west. So forget the amphibious warfare angle.
And a US-led invasion seems unlikely given past Chinese sensitivity to American military operations so close to China's border.
No, to me this makes the most sense when you consider that the most important thing North Korea spends money on (after goodies for the elites) is their nuclear program.
North Korea seems to seriously want nuclear missile-armed subs. These would be the most survivable nuclear weapon given American and allied air superiority which could strike North Korean land-based nuclear weapons at will.
But the North Korean navy is ill-equipped to defeat the American navy and our allies at sea.
Nor is the North Korean navy going to to send ballistic missile subs very far east to get within range of America's west coast. If our Navy can't find and sink those old subs on the way to their launching point, I'd be shocked and outraged.
The objective that makes the most sense is that North Korea wants nuclear-armed submarines and to protect them North Korea wants to establish a Cold war-era concept of a "sub bastion" in the Sea of Japan in which North Korean subs protected by sea-going and land-based military assets can survive long enough to launch nuclear missiles.
If North Korea's non-nuclear-powered ballistic missile boats go to the Sea of Japan and just sit somewhere quietly (could they rest on the bottom?), we'd have to penetrate the bastion to find and sink the subs. And elite ground forces would help protect any subs in port getting ready to go to sea from special forces raids.
And if North Korea has working nukes, the money for goodies for the elites will flow in, I dare say.