Ambassadors theatre, LondonIan Shaw plays his film star father Robert in this behind-the-scenes tale of Steven Spielberg's blockbuster In the waters off Martha's Vineyard in 1974, three men in a boat " droning Roy Scheider, grizzled Robert Shaw and neurotic Richard Dreyfuss " are waiting for a mechanical shark named Bruce to start working. The second-best thing about The Shark Is Broken, a three-hander by Ian Shaw (s…