Most Americans (62 percent) believe that men go to the beach to watch women, says a recent poll by Audits & Surveys, Inc. Thirty-six percent think women go there to be watched.

But the mission of beach-bound people who really want to have fun, claims Michael Dipersio, is to build sand castles. The author of Castles in the Sand: 10 Projects That Can Be Built in One to Six Hours (Perigee, 107 pp., $6.95) built his first castle--for his daughter--in 1969 on the New Jersey shore. He went on to become a "master castler" with such classics as The Alamo, The Colosseum and The Great Wall of China.

The "basic fantasy castle" can be built in one hour; a dazzling "sunburn special" requires a full-day's work. Because a "really good sandcastle always attracts onlookers and admirers," he writes, it's a lure for other sunbathers.

Dipersio's standard sandcastling tools: shovel, plastic pail 8- to 10-inch diameter, plastic cup 4- to 6-inch diameter, funnel, 6- and 12-inch rulers, spatula. For "fancy trim": melon baller, yardstick, wineglass, flowerpot, popcorn bowl.

Among his other grains of wisdom:

* Keep a lookout so an admirer won't topple the Eiffel Tower.

* Bring a camera to capture the completed structure: "There's not much you can do about the inevitable."

* Remember, the fun is in the building.