One of the largest cruise ships in 1985 was the 46,000-ton Carnival Holiday. Ten years ago, the biggest, the Queen Mary 2, was three times as large. Today’s record holders are two 225,000-ton ships whose displacement, a measure of a ship’s weight, is about the same as that of a Nimitz-class aircraft carrier. Cruise ships keep growing bigger, and more popular. The Cruise Lines International Association said that last year its North American cruise line members carried about 17 million passengers,…