What's nine stories, sleeps 360 people, and floats?

For sale: ready-made hotels delivered to your door . . . or at least your dock. A leading Japanese company is offering to construct hotels in its shipyard and float them to any coastal or accessible riverside site anywhere in the world.

Nippon Kokan K.K. says the proposal has already drawn many interested inquiries, particularly from developing countries short of first-class construction equipment and materials, as well as skilled workers for sophisticated hotel construction.

Nippon Kokan and the Tokyo Construction Company have spent two years solving the technical problems of their hotel-for-export plan. It is basically an extension of existing technology that has seen various types of factories and electric power stations built in Japanese shipyards and towed by tugs across the oceans to the customer.

The new application calls for a massive barge to be built in a dry dock just like a conventional ship, providing the foundations on which a multistoried hotel can be constructed virtually as if it were on land. After completion, sea water is pumped into the barge to lower its center of gravity and make the structure stable. It can then be towed off to its final resting place at a steady five knots per hour, defying the worst of storms, Nippon Kokan says.

The standard design will be a nine-story hotel 79 feet wide, 337 feet long, and 106 feet high. It will have about 360 guest rooms, the usual eating and shopping facilities, and an outdoor swimming pool. Variations on this are possible as long as they do not exceed the barge's carrying capacity, a Nippon Kokan spokesman stressed.

''A big advantage of this system is that land shortage will no longer be a handicap in hotel construction,'' he said. ''It (the hotel) can be moved around to different sites to accommodate conventions or expositions, or to a variety of different tourist locations, as well as shifted between summer and winter resorts where justified by changes of the season. Alternatively, there is no reason why the hotel could not end up as a conventional, land-based one. . . .''

Nippon Kokan's major rival, Ishikawa-jima-Harima Heavy Industries Company Ltd. (IHI), actually pioneered the technique in the mid-1970s when a floating pulp mill as big as an oceangoing passenger liner was towed to Brazil and moored on the banks of the Amazon River hundreds of miles inland.

Since then, barge-based power stations have been built and floated away to sites in Thailand, the Philippines, and Bangladesh. Sea water desalination plants and floating apartment blocks for migrant workers have gone to the Middle East.

For the Japanese shipbuilding industry, such high-technology structures offer a welcome new route to continued profits in an era of shrinking world orders for ships and increasing competition from newly emerging high-quality shipbuilding nations like South Korea and Taiwan.

The new Japanese idea has yet to achieve its full potential, however. Industry experts had expected the best market would be the so-called third world. But developing countries have not had the money to pay for such expensive luxuries since the world economy went into decline after the second oil crisis of 1979.

Now, there is the chance for such countries to obtain export financing from the Japanese government, under a scheme for revitalizing the shSpbuilding industry devised by the Ministry of International Trade and Industry (MITI).

An IHI official explained that, in ordinary circumstances, conventional on-site construction of factories, hotels, or whatever was still much cheaper. ''The floating factory, however, becomes cost effective,'' he asserted, ''when it is sited in an inaccessible or totally undeveloped area without good road or rail links and reliable electricity supply.''

MITI reports a number of countries in Asia, the Middle East, and South America are interested in having barge-based power stations built in Japanese shipyards. Ecuador wants a paper-pulp plant, while the Philippines and Bangladesh are said to be interested in floating factories to collect and distribute grain.

Still at the blueprint stage is a project for floating airports, eliminating the problems of noise pollution to neighboring communities that shut down most Japanese airports at night.

The idea was considered by the city of Osaka. Located five kilometers (about three miles) offshore, the airport's two runways and supporting facilities would have sat on cylindrical buoys in 200 rows of 50 each and been secured by pylons sunk into the bedrock. In the end, however, Osaka went for an airport built on reclaimed land.

Another ambitious proposal for Japan itself is a floating highway to entirely circumnavigate the Japanese coastline, leaving more land for residential and industrial use.