Doomwatch - Season 1 Episode 3 Burial at Sea

When a lifeboat crew discovers a luxury cruiser adrift in the English Channel, they find on board are a group of wealthy pop stars and their girlfriends, one of whom is dead. Doomwatch investigate, and discover that the death was not from drug abuse as was previously suspected; instead they begin to uncover the facts behind the dumping of a deadly chemical warfare compound in a deep sea trench, which seems to be far from dormant.