In a way, it would be a terrible shame if the existence of the Loch Ness monster was ever proved or disproved. Just think of all those charmingly speculative stories that would be absent from the London Times. Now comes word that an enterprising Inverness boat company is offering insurance against possible attack by Nessie. Jacobite Cruises, which runs two boats on Loch Ness, admits an onslaught by Nessie is improbable. "But," says a spokesman, "we are also covered if there is a regular accident."