"What do you with 300 toy boats?" Peter Purpura asked last night.

It was a question that the wife of Malcolm Forbes must have asked many times as the collection of toy boats mounted in the pantry and playroom of her suburban New Jersey home.

Last night Purpura, curator of the National Geographic Society's Explorer's Hall, had the answer as the exhibit of the Forbes collection of toy boats opened at the Society, 17th and M Streets, NW.

What Purpura had done with more than 300 toy boats is turn them into a delight for the young in spirit of any age. They glide on crystal-clear marbles in an old bathtub and they float on packing bubbles to the lonely sounds of seagulls and foghorns.

The reception and dinner last night was a preview of the show "A Century of Toy Boats," which opens today.

The collection was assembled by Malcolm Forbes, the business magazine publisher, and his son, Robert, a photographer who has collaborated on a new pictorial history of toy boats.

"The toy boats accumulated in our pantry and playroom," the elder Forbes said last night. "My wife's reaction when they were leaving was that she didn't care where they were going as long as they were going."

None of the toy boats that were playthings for his son, Robert, has survived.

"I sank them all," Robert admitted ruefully, "in the bathtub, the swimming pool, the stream . . . I shot BB gun pellets at the plastic boats and the tin boats, which were more fun to sink. There were the first flotilla of the Forbes collection."

What remains for the exhibit are the boats later collected by two generations of Forbeses. Their collection is worth more than a quarter-million dollars and includes a silver paddle steamer made in 1913 by Faberge as a present for the only son of Czar Nicholas of Russia.

There are river boats, ocean liners, warships, speedboats, and submarines, all power toy boats that were made to be handled and played with in contrast with more delicate ship models.

Purpura, the exhibit's designer, has added the sights and sounds of the sea. And there is a Charles Addams cartoon showing little, well-behaved boys directing their sailboats around a pound while a mean little kid gets ready to launch a black submarine. The elder Forbes, who is a hot-air ballon enthusiast, wrote the words that introduce exhibit:

". . . toy boats in the Forbes Family were more prized than electric trains or other playthings. We were forever sailing them in nearby brooks and streams and in summer on lakes or oceans . . . Not a one survived to be part of this collection. But the memory of them and joy of them account for its formation.