GETTING THERE:In 21 days, our group visited some of the most remote back country of Belize, Guatemala and Mexico. Given the logistics of planning such a complex trip -- the security of settlements and accommodations, the difficulty of buying and preserving food supplies, the language barrier (hardly anyone speaks English between Tikal and Palenque) and the fact that the unpredictable and ever-changing Usumacinta is not to be treated casually -- guides are recommended. We put ourselves in the hands of Far Horizons (P.O. Box 1529, San Anselmo, Calif. 94960, 415-457-4575). The trip cost $3,750 ($3,950 in 1988) per person, including round-trip air fare from Miami, all meals, hotels, boat and guides. River travel was subcontracted to individual boatmen in Guatemala and to Far Flung Adventures (P.O. Box 31, Terlingua, Texas 79852, 915-371- 2489), the rafting outfitters who deftly handled the river passage from Corozal to Palenque.

You do not need a visa to enter Belize or Mexico, but you do need one for Guatemala, obtainable free of charge from the Embassy of Guatemala, 2220 R St. NW, Washington, D.C. 20008, 745-4952.

WHERE TO STAY: We stayed in a wide variety of places, including:

Tikal Inn, Tikal, Pete'n, Guatemala. Thatched bungalows for two, $10 per person; breakfast is $2, dinner $3. Reservations are required.

Hotel Maya International, St. Helena, Guatemala (near Flores, en route to Sayaxche). This is a series of large thatched buildings of double rooms with baths, including a central dining hall. These are cantilevered over the waters of Lake Peten Itze and connected by wooden walkways. Reservations are required 15 days in advance through Servicios Turistices del Pete'n, 3 Calle 10-58, Zona 10, Guatemala City. Cost is $12 per person; meals are extra.

Petex Petun Camp, c/o Panamundo Guatemala Travel Service, S.A., 7a Avenida 14-14, Zona 9, Guatemala City. Prices vary depending on group size. Reservations are required.

Hotel La Canada, Calle Merle Greene No. 14, C.P. 29960, Palenque, Chiapas, Mexico. Spacious double rooms with hot showers run $14 for two, $12 for one person. Open-air dining; breakfast is $3, dinner $6.50.

WHAT TO TAKE: Clothing should be of washable cotton, and quick-drying. Long pants, long-sleeved shirts, socks and sturdy walking shoes are a must. In addition, the following items are recommended:

An extra pair of quick-drying sandals or sneakers, for beach wear and boat travel.

Sunscreen, sunglasses and a hat with a brim or visor.

A bathing suit, shorts and T-shirts.

Insect repellent -- the mosquitos are vicious, especially in deep grass and marshy areas.

Water purification tablets.

Camping supplies, including a canteen, flashlight with extra bulb and batteries, biodegradable soap, poncho, tent and lightweight sleeping bag (or a sheet sewn together like a sack), eating utensils, a cup and plate. Everything should be carried in a soft-sided dufflebag to fit in rubber "dry" bags supplied by the rafting crew. You would be wise to take smaller waterproof bags for camera equipment and binoculars, and a daypack for jungle hikes.

Credit cards and traveler's checks are not very useful in this part of the world: take small bills of U.S. cash, which can be easily exchanged. HEALTH: Malaria is endemic in the region. Under a doctor's guidance, begin taking malaria pills prior to your departure and continue them for the prescribed length of time following your return. Be sure your typhoid shots are up to date.

To be safe, do not drink unboiled tap water -- although our group used a water filter with success.