COLOMBO—A cargo ship carrying tonnes of chemicals sank off Sri Lanka’s west coast, its navy said on Wednesday, and tonnes of plastic pellets have fouled the country’s rich fishing waters in one of its worst-ever marine disasters. The government on Wednesday suspended fishing along an 80-km (50-mile) stretch of the island’s coastline, affecting 5,600 fishing boats, and hundreds of soldiers have been deployed to clean affected beaches. The Singapore-registered MV X-Press Pearl, carrying 1,486 containers, including 25 tonnes of nitric acid along with other chemicals and cosmetics, was anchored off Sri Lanka’s west coast when a fire erupted onboard after an explosion on May 20. Flaming containers laden with chemicals tumbled into the sea from the ship’s deck as emergency crews sought to contain the blaze over the ensuing two weeks. The craft began to sink early on Wednesday, and a salvage crew tried to tow the vessel to deeper …