Royal jewel plot, The. "Mysteries at sea."

Sinopsis
Alice and Sonny are excited for their summer trip, sailing on the luxurious yacht the Lady Rose. They're even more excited when they find out the King of England is going to be onboard too! There's also a precious jewel on the boat - a rare opal, kept in a tank protected by a poisonous octopus. Until one morning, Alice discovers that the opal and octopus are both missing. The search is on to retrieve the priceless gem, before the ship reaches its destination, and everyone on board finds themselves in serious trouble. As Alice and Sonny's investigations take them closer to the truth, they realise that everyone on the Lady Rose seems to have a secret, maybe even the king himself.