Port of No Return — Ruth McCarthy Sears

The cruise was to be a special treat for the three college chums before they settled down to the realities of the workaday world. But it was not long after the ship's sailing that they discovered the body of one of their bon voyage guests in a lifeboat and began to develop distrust of their fellow passengers, a surprising number of whom seemed to have dual identities, secret lives and a penchant for intrigue.