DEAD IN THE WATER : LOOKING FOR A SHIP <i> by John McPhee (Farrar, Straus & Giroux: $17.95; 242 pp.; 0-374-51690-1)</i>

This is the latest installment in John McPhee’s portrait of America, in this instance a visit with the U.S. merchant marine.

McPhee has become something like the Farmer’s Almanac--annual and taken for granted. He is first in the New Yorker, then somewhat later the pieces appear between the covers of a book, and we sometimes half-listen to the steady voice because it is so dependable. But when we turn to him we become absorbed because we learn things we did not know.

The merchant marine is dying (shrinking from 2,000 bottoms to 400 in the ‘80s) as American shipowners flee to foreign registry and as foreign fleets, including the Soviets’, gobble up our cargo. The men who still work our surviving ships are getting old and are often in their 50s and 60s. They seldom can find work--maybe half the year, if lucky. Thanks to a once-strong union, they are paid a lot of money ($30,000 to $60,000 a year) for the days they do work.

All these facts are crisply conveyed in the opening section of “Looking for a Ship” when McPhee decides to ship out with Andy Chase, a young man who is a descendant of every navigator’s god, Nathaniel Bowditch.

The key fact of life for an American sailor is having a “killer card.” The longer a man has been stuck on land during the calendar year, the more potent his position in the hiring hall, until by late December, his union card becomes a lethal weapon in the scramble for a berth. So Chase spends a lot of idle time sitting around in Charleston waiting for a job. Under the rules, a sailor cannot call a hall and inquire; he must be physically present. Men seeking ships spend a lot of time driving from port to port chasing hunches about work.

This is just one of many quirks of the merchant marine. And that is a good part of the seduction of the book. It is a strange world where a man does not stay with a ship beyond one voyage (and sometimes for less if during trip he uses up his six-month sea limit), a culture where his fellow union members are, between jobs, viewed as enemies fighting for the same slots and not to be confided in. This creates a climate of clipped statements.

“Andy had the ship: the Stella Lykes . . .

“Dauksevich, surprisingly, said, ‘Where are you going?’ ”

“ ‘West coast of South America,’ Andy said.

“Dauksevich said, ‘Don’t get the clap.’ ”

Once at sea, the personalities appear, and they are the stuff of sea tales, people often larger than life, and almost always competent. Captain Paul McHenry Washburn is in his 60s, but began life as a runaway from a respectable home, hoboed, worked as circus freak, fought in the ring for money and wound up at sea as a laborer, an ordinary seaman.

The sea bewitched him. He was trained up by captains with appropriate names--Leadline Dunn, Terrible Terry Harmon, Dirty Shirt George Price--and now when not at sea, he lives in a Florida condo, plays golf obsessively, feeds the birds and knocks down about $100,000 a year.

Like many mariners, he loves the life of ships and does not love the ocean. McPhee drives home the perils of the sea, and this obvious fact comes as a surprise. It is not just the tales of men falling overboard, of giant sea creatures biting sailors in half. It is the constant fear of sinking, the litany of global reports of daily disasters on the waters. Much as we have forgotten the existence of the merchant marine, we have forgotten the terrors of the deep. One sailor in the book expresses the rage he feels at home on land when a weather report will say that a storm has gone “safely out to sea.”

The mechanics of running a large ship also have been forgotten--the engine room at 140 degrees, the tricks of navigation, the exotic vocabulary, the constant possibility of collision or of running aground. Surprisingly, the ships have almost no crew, a consequence of American owners slashing payrolls to prop up profits. A Navy vessel might host a crew of 250, where a commercial vessel of comparable size makes do with 20 or 30 hands. For men of the Stella Lykes, this is in some ways a matter of pride; but inevitably during a voyage to Chile and back, the talk returns again and again to the death of the merchant marine, the ending of a way of life.

As the number of ships shrinks, the number of openings declines; and for young men coming up like Andy Chase, it is much like the U.S. Army after the Civil War: a reduced force with too many officers serving below their one-time rank and no chance of advancement. None of this seems to register with the men. True, they complain, but they do not seek other lives and jobs. For them there is no substitute for a ship, a perfectly functioning organism with a specific purpose: the voyage.

McPhee discovers one other thing we have all forgotten: pirates. In places, the sea lanes are under siege from pirate vessels, and the Stella Lykes is boarded by thieves as it approaches Guayaquil, Ecuador. There is almost nothing the crew can do about this. The ship is large and intricate; men can board, steal and leave before the handful of sailors even knows of their presence.

“Understand: this ship is about the length of Port Authority Bus Terminal, Rockefeller Center, Pennsylvania Station, Union Square (or Dodger Stadium, including the parking lots). To berth her, you need almost three city blocks . . . She carries a crew of 34. Thirty-four highly trained SWAT troops would have a hard time defending Rockefeller Center . . . The ship might as well be an open city.”

In the bookkeeping of modern commerce, it is cheaper to endure pirates than employ a crew big enough to protect the cargo. The pirate raid comes off as frightening for the crew and yet somehow very matter-of-fact, a thing like the weather to be cursed but endured.

And that is true of the merchant marine, a world where time is divided into tidy watches, where the ship plows ahead day and night, where things generally work properly and in harmony. The book ends with the Stella Lykes’ engines down, the big vessel helpless, like the American merchant marine, on the water.

“The clouds are very dark off starboard quarter. With our lemons and lollipops and terry-cloth towels, our three thousand cases of wine, with our 90 drums of passion-fruit juice, our onions, umbrellas, bone glue, and balsa wood, our kiln-dried riata pine, with our glass Nativity scenes and our peach chips, we are dead in the water.”

In the big scheme of things, it may not matter if all the American bottoms finally leave the oceans. Maybe commercial shipping makes no sense for the United Sates. But McPhee half-convinces us with one voyage that somehow we will be a lesser people if such a thing comes to pass. We will have surrendered our access to a great source of fear and mystery.