When the Japanese sent a "floating fair" ship to expand exports to the United States 20 years ago, who could imagine that a similar ship would be used to promote sales of American products to Japan?

This floating department store is called "Boatique America" and will cruise 13 Japanese ports during the next two months as the "Sakura Maru" did in U.S. waters then.

Sponsored by the Commerce Department and backed by the Japan External Trade Organization, the ship carries 8,000 items ranging from beef and bathing suits to toys and towels.

U.S. Undersecretary of Commerce Luther Hodges Jr. launched the 13,000-ton ship in Tokyo Bay. Hodges said there are two goals for the voyage. One is to introduce made-in-America products to the vast Japanese consumer market. The second "is to increase the export consciousness of American manufacturers."

"Our country had not participated in foreign trade to the extent that Japan or any other trading partner had. The U.S. now faces new economic reality and must seek new markets . . . Must trade abroad," Hodges said.

The whole idea began in the spring of 1978 when a Japanese trade mission headed by Yoshizo Ikeda, chairman of the Mitsui trading group, toured the U.S. to increase Japanese imports.

Ikeda proposed at a businessman's luncheon that the Japanese could provide use of the Japanese trade fair ship, which was then idle. The Japanese has used the Skaura Maru as a floating marketplace around the world, including the U.S., from 1956 until 1972 when it was replaced by a newer, bigger vessel.

Since then, the Commerce Department has enlisted 145 companies to join the project to market 8,000 items worth $20 million at retail. American government is spending $2.2 million for the floating promotion of American goods, many new to Japanese consumers. And the Japanese have come up with $3 million of their own to support the program. Much of this will have to come out of the sales abroad the "Boatique America" during its two-month cruise.

Major items of interest are beef, toys, women's and children's apparel, leather products and sporting goods. The price will be 10 to 40 percent less than the equivalent products sold in Japanese department stores but slightly higher than American prices.

"Boatique America" has received wide publicity in Japanese magazines, newspapers and television. An estimated 500,000 Japanese are expected to board the vessel during its cruise.