FOR ALL THE WARNINGS about health hazards, deadly mamba snakes and other supposed perils, the last six days have turned out to be a relatively pleasant experience for the huge entourage that is following President Carter on a journey to four developing nations.

For the last two nights, the American party, numbering several hundred, has been housed in the sleekly modern Eko Holiday Inn hotel, a few coastline. Saturday night, and the first hours of Sunday, the Americans frolicked in the hotel swimming pool, into which a fully clothed Sam Donaldson of ABC television was thrown while his colleagues sang "We Shall Overcome."

To be sure, the American party has encountered some of the common problems of the Third World. In Caracas, for example, a power outage coincided exactly with the arrival of the Americans at their hotel. Here in Lagos, the breakdown was human rather than mechanical - precisely at the moment that U.S. reporters arrived at the hotel, anxious to began sending their stories to the United States, the local telex operators decided they were finished for the day. But these have been relatively minor inconveniences compared with what was expected when the U.S. party left Washington last Tuesday.

It was warned of all kinds of perils, among them the mamba snake said to inhabit Liberia, where the President will touch down briefly today before returning to the United State. So far no snakes, mambas or otherwise, have been encountered.

This trip also contrasts well with Carter's last journey overseas, which took him to seven countries in nine days, exhausting the president and almost everybody who accompanied him. Learning from that experience, White House officials constructed a less hectic schedule for this trip, including in it a free evening to enjoy the splendors of Rio de Janeiro.

A first-time visitor to the Third World comes away with two strong impressions. The first is the displays of military power in the countries that the president has visited.

Whether in Venezuela, Brazil or Nigeria, Carter has been surrounded by soldiers armed with automatic weapons whenever he has appeared in public, dwarfing any kind of security arrangements seen in Washington.

The Brazillians, however, may have gone too for in flexing their military muscles. They sent two jet fighters to intercept the two press planes as they flew into Brasilia.

The second impression is that of all the Third World's well publicized problems, there is one with which Americans can easily identify - traffic jams.

It can take five hours or more to get the few miles from central Lagos to the city's airport. It was not much better in South America.

The Nigerians were struck by the size of the press contingent that accompanied Carter. A columnist for the Lagos Sunday Times, Mvendaga Jibo, offered this explanation:

"It is now well known that some American journalists are part of the official intelligence-gathering apparatus. It is, therefore, conceivable that some of the journalists have some to get the information needed fo fill in a gap in the knowledge (that) the Pentagon or the CIA has about us. And Americans are big spenders when Intelligence information is available for a fee."

"I HAVE ALWAYS wanted to be someplace where the only safe thing to drink is beer," an American reporter remarked the other night.