Walt Disney World's Epcot Center is undertaking more than $800-million worth of new projects, the most recent of which is "The Living Seas," a 6-million-gallon ocean pavilion that opened two weeks ago.

Designed to showcase a variety of sea life, the pavilion displays the latest technologies for undersea exploration and features demonstrations by research teams working with dolphins.

Other projects underway at the Disney entertainment center in Buena Vista, Fla., near Orlando, are a water-thrill park, "Splash," now being designed and expected to open in two years; and the fourth Disney-owned-and-operated hotel at the Florida amusement park, the 900-room Grand Floridian, with construction slated to begin this year. Also under development on the extensive Florida site are a $330-million movie studio/tour attraction, scheduled to be finished late next year, and a $265-million, 2,300-room convention hotel complex that is also due to open in 1987.

The Living Seas pavilion is sponsored by United Technologies Corp., which underwrote most of the more than $50 million reported cost. However, other projects will be subsidized -- at least in part -- by recently increased ticket prices.

Disney World admission prices, both to the Magic Kingdom and to Epcot Center, were raised in June and again in November, with increases totaling about 19 percent. For example, a three-day pass (the most popular ticket) rose more than 10 percent to $53.50 for adults and $45.50 for children on Nov. 24. The boosts were a direct result of the capital-improvements pro- gram, according to Dick Nunis, Disney World president.

The self-contained saltwater environment of The Living Seas simulates a living Caribbean coral reef. It is the seventh major attraction in Epcot's Future World and is 200 feet in diameter and 27 feet deep. Completely enclosed -- in a 170,000-square-foot circular building -- to facilitate light and temperature control, it had an initial population on opening day, Jan. 15, of more than 2,000 sea creatures -- sharks, barracuda, dolphin, angelfish, snappers, manta rays and parrotfish -- a number that is expected to double as more fish are added in coming months.

To ensure authenticity, plans for the pavilion were reviewed by a board of advisors that included Gilbert Melville Grosvenor, president of the National Geographic Society, and Robert Ballard, head of the Deep Submergence Laboratory, Woods Hole Oceanographic Institution of Massachusetts, the American leader of the recent expedition that discovered the wreck of the Titanic.

The centerpiece of the pavilion is Seabase Alpha, which represents a research center on the ocean floor in the 21st century. Divers ascend through an air lock and a water-filled cylinder to the "ocean" above.

The exhibit also includes two theaters, veterinary laboratories, research facilities for experiments in communicating with dolphins, systems for transporting guests through the undersea world, computer systems that inform visitors about what they are viewing and displays on the productive, yet environmentally protective, use of undersea regions.

The Living Seas journey begins in one of the two 185-seat theaters with a short film about the ocean, then visitors enter a "hydrolator" -- a device that gives the illusion they are descending through the sea in an elevator. Next they board a continuous train of "seacabs," which travel through an ocean-floor tunnel past wide windows overlooking the reef. After arriving at Seabase Alpha, they see new ocean surveillance and management technologies. They can speak directly with the researchers/divers by radio and manipulate remote-control underwater robots.

At the Coral Reef Restaurant, customers dine while looking directly into the reef through windows 50 feet long, 8 feet high and more than 8 inches thick.

TRAVEL ADVISORIES: The U.S. State Department has recently issued a number of advisories for world travelers covering subjects ranging from a fuel shortage to currency controls. Among them:

*Americans planning to visit Tanzania should be aware that the African country currently is suffering from a severe fuel shortage. Areas outside Dar es Salaam, the capital, are particularly affected, including the northern game parks, the State Department noted. Some tour operators reportedly have been forced to curtail their operations.

Persons driving in Tanzania at this time should be certain they have an adequate supply of fuel. Travelers who have been booked on organized tours may want to make sure the tour operator is aware of the problem, the department suggested.

*Hotel rooms in Belize City will be scarce until mid-April, due to the filming of a movie. Travelers to Belize are advised to reconfirm reservations before leaving the United States. Accommodations on the offshore islands are unaffected.

*Visitors to Guatemala who are planning trips outside major tourist areas -- such as Guatemala City, Antigua, the north shore of Lake Atitla'n and the Mayan ruins at Tikal -- are warned that "encounters continue between Guatemalan security forces and guerrillas."

The affected area extends roughly 31 miles into Guatemala along the border with Mexico (except for Pete'n, in the far north). Encounters also continue in the highlands on the south and east sides of Lake Atitla'n. The State Department advises travelers to avoid these areas at all times.

Entry to Panajache'l and the north shore of Lake Atitla'n should be made via the Los Encuentros-Solola' turnoff from the Pan American Highway. Land entry from Mexico should be via the Pan American Highway crossing at La Mesilla/Ciudad Cuauhte'moc. Nighttime intercity road travel anywhere in the country is inadvisable.

*Strict currency controls have been imposed by the military government of Nigeria, and violations are punishable by arrest, prosecution by military tribunal and prolonged detention. All new legislation at the state and federal level is by decree; the State Department recommends that U.S. citizens traveling on business to Nigeria "determine the relevance" of these decrees to their activities.

The country's land borders are closed, and all visitors must arrive and depart by air or sea. U.S. citizens are advised to register with the U.S. Embassy at No. 2 Eleke Crescent, Victoria Island, Lagos (phone 610-050), or the American Consulate General at No. 2 Maska Rd., Kaduna (phone 213-074).

More information about these advisories and other areas is available from the State Department's Citizens Advisory Center, 647-5225.

IN GRIMM FOOTSTEPS: The bicentennial observance of the births of fairy-tale collectors Jacob and Wilhelm Grimm is continuing this year in West Germany.

The Brothers Grimm were born in the town of Hanau, in the state of Hessen, just east of Frankfurt -- Jacob in 1785 and Wilhelm in 1786. The celebrations throughout the country include fairy-tale festivals, fairs, concerts, parades and exhibitions.

The two brothers lived and worked most of their adult lives in Hessen, and reminders are everywhere. A monument stands in the Neustadter market square of Hanau. In nearby Steinau, where they spent their childhood, fairy tales are dramatized every weekend in the 140-seat Steinau Marionette-Theatre. In Kassel, where they worked as librarians, the Brothers Grimm Museum exhibits first editions, manuscripts, family portraits and Grimm books translated into 51 languages.

And throughout Hessen visitors can see castles and palaces, parks and spas, mountains and forests (nature reserves cover almost a third of the state) that recall some of the most famous stories the brothers compiled. Much of the countryside appears the same as it did when they were alive, and the same traditions and costumes can be found along Germany's "Fairy-Tale Road," which is actually a number of routes running northward from Hanau to Bremen.

A new brochure on Hessen, subtitled "The Home of the Brothers Grimm," and including a map, is available free from the German National Tourist Office, 747 Third Ave., New York, N.Y. 10017.