The return of Halley's Comet will launch many serious comet watchers on far-flung journeys in the next few months. The comet generally will be visible around the world until the end of May -- except from the last week in January to the end of February, when it will be either behind the sun or too close to its glare to be seen. The best possible glimpses will be in the Southern Hemisphere, where the comet will be the brightest. Among the prime vantage points, and some of the special cruises and tours planned for comet viewing:

*Miami to the Florida Keys: Dade and Monroe counties have proclaimed themselves "Comet Country U.S.A." Miami's Space Transit Planetarium is sending vans equipped with telescopes outside Miami and in the Keys, through April, for free observation by the public. Key West will hold a month-long Halley's Comet Festival '84 beginning March 13.

*Caribbean: Home Lines' 10-day "Sky Watch" cruise sails March 16 from Port Everglades to San Juan, St. Thomas, Martinique, Barbados and Antigua. Lecturers will be aboard and early-morning viewing sessions will be held at sea. Cost: from $1,810 to $3,570, including round-trip air fare from Washington and other cities. (All prices are per person, based on double occupancy.)

*Mexico: Holland America Line offers a March 1 departure from San Francisco for a 14-day cruise along the Mexican west coast. Experts will lecture and help passengers study the comet with binoculars while at sea. Cost: $1,945 to $3,395, including round-trip air fare.

*Brazil: Sun Line's 19-day "Halley's Comet/Rio" cruise embarks March 1 from Fort Lauderdale. The vessel will visit Bele'm, Bahia, Rio de Janeiro and other ports. Cost: $3,450 to $6,660, including return air fare from Rio to Miami.

*Peru: Mountain Travel's 10-day tour begins March 28 in Lima and provides four nights of observations in Arequipa. Cost: $1,290, excluding air fare.

*Australia's Outback: Astronomy Tours International's 15-day Halley's expedition includes the chance to visit Alice Springs and Ayers Rock for 60 hours of viewing in the Outback. The departure is April 5 from Los Angeles; cost: $3,400.

*South Pacific: Royal Viking Line has scheduled four comet cruises from February to April, but only one currently has space -- the March 12 sailing from Sydney. During the 14-day journey to Auckland, New Zealand, there will be sea and land viewing -- from several different Pacific islands -- with guidance from an astronomer. Cost: $2,828 to $10,220, excluding air fare.

*Asia: On Pearl Cruises of Scandinavia's 18-day "Bangkok, Bali and Beyond" Halley's cruise/tour program, passengers will fly from Los Angeles and San Francisco to Bangkok on March 17. The ship will be positioned around the equator, and viewing will be at sea with binoculars and on Bali, where telescopes will be set up. The cruise ends in Singapore; a March 30 cruise reverses the route. Cost: $3,424 to $8,494, including air fare from Washington and other cities.