Sailors have long alleged that Dragalge feed primarily on the remains of sunken ships, and that only when a waterway becomes well-traveled can the local Skrelp grow large enough to evolve. This is not all that close to the truth; although Dragalge will occasionally feast on the hulls of shipwrecks, and far more often on the cargo, they are more than capable of subsisting solely on wild pokemon.
In truth, Dragalge sink fishing boats for the same reason they attack other water predators - they rightly consider them competition for scarce prey. They attack other vessels because they never learned to distinguish fishing boats from passenger or cargo ships; should they ever realize the difference, they would still target human-piloted boats, as they would target any other strange species of water pokemon.
Because of this tendency for destruction, humans have often introduced the few Dragalge they manage to capture or at least influence into areas far removed from their natural range, for humans have a long history of warfare, and are more than willing to set a living natural disaster on their enemies for the sake of territory or prestige. As a tool of warfare, Dragalge are far more effective than naval mines, but have roughly the same function: rendering a harbor or particular area of ocean which the Dragalge designates as its own territory impassible by boats.
The problem with using Dragalge is that, like privateers, Dragalge do not end their reigns of terror simply because kings sign a treaty. Large areas of coastline became impassable for decades after wars until the introduced Dragalge finally died off. And in some instances, a breeding population of Dragalge established itself in a harbor or shipping lane, and the affected waterway had to be permanently abandoned.