Decked (1992) by Carol Higgins Clark

The daughter of a successful mystery writer and a P.I. with pizzazz, Regan Reilly is attending her class reunion at St. Polycarp's England, when the long-dead body of her former roommate turns up under the bushes. It's a case Regan would love to solve, but a prior commitment puts her on a transatlantic cruise. She shouldn't have fretted. The clues to the crime are following her on the Queen Guinevere. Here, on a ship awash with secret lovers, a fortune hunter, a jewel thief and an assassin, Regan is sailing home - and into the hands of a young girl's killer.