Thar She Blows

''To watch humpbacks here you merely have to be able to tell time,'' says Greg Kaufman as the Aikane II heads away from its berth in Lahaina's picturesque harbor. ''Think of our bow as 12; at 3 is the island of Molokai. The stern and Maui are at 6 o'clock; and 9 is Lanai, there on our left. It's as easy as that. Now, when any of you spots anything, just yell out the time - then we'll all know where to look.''

What we are looking for, it seems, is the same thing every whaler since Captain Ahab has used to pinpoint the location of whales - a pear-shaped ''blow, '' a spout of vaporized water that is actually the whale's exhalation.

We are not whalers of Captain Ahab's ilk, however. We are on the Aikane II in the waters off Maui not to catch them, but to watch them.

Whale watching often can be a waiting game; this afternoon it isn't. Almost immediately someone sees something, and the yell rings out: ''Ten o'clock!'' About one-quarter mile off the port bow a translucent plume of water forms a clearly visible part of the seascape. It is followed by the slap of a patent-leather black tail fin.

The movement is graceful, as if it's part of an aquatic ballet; we're doubly sure that it is when moments later another whale, swimming with the first one, blows and whacks its tail on the water's surface too.

This is easy, we think, and we're just about to congratulate ourselves on seeing our first humpbacks when Mr. Kaufman, who has been watching the action through binoculars, speaks again: ''Don't get too excited,'' he says. ''Those two are pilot whales. They often swim with humpbacks, but they are a lot smaller - only half the length of a full-grown humpback's flipper.''

Mr. Kaufman, president of the Pacific Whale Foundation, should know. When he isn't guiding people through this winter habitat of the humpbacks, he studies their behavior.

Whales, though they live in the seas, are not fish, but air-breathing, warm-blooded mammals that give live birth. In fact, that's why many of them spend the months from November to May off west Maui.

People here call these the ''bedroom waters'' of the humpback whale. The Bering Sea between Alaska and Siberia makes a good feeding ground; but when the humpbacks are ready to calve, they head toward these more temperate climes. The warm Hawaiian sea is safe for mating, and it pampers the blubberless newborn whales until they can gain weight (which they do at about seven to nine pounds an hour).

At birth a humpback is 6 to 10 feet long; three years later, an adult female may measure 45 feet.

We get these bits and pieces of information from Mr. Kaufman as we sail, for he hopes that the more we know about these mammals, the more we will feel we have a personal stake in their future.

One of the stated objectives of the Pacific Whale Foundation is ''to prevent the biological and/or commercial extinction of whales. . . .'' Indeed, the uniform of the day seems to be the blue Pacific Whale Foundation T-shirt bearing the inscription, ''Extinction is forever.'' It points up, in graphic fashion, the fact that the group of humpbacks that winter here are a mere 7 percent of the numbers that existed in this area at the turn of the century.

''There are only 850 humpbacks in all of the North Pacific today. So when you see one, you've seen 1/850th of the entire population,'' Mr. Kaufman says.

We may only come within 100 yards of the whales. That's the law for commercial whale-watching vessels. It is meant to keep boats from harassing them. ''Of course, there are no rules about whales harassing people,'' he adds, ''and sometimes they'll come so close to us, we think they're people watching.''

In fact, this particular cruise is on the anniversary of just such an occasion, and one of the passengers who had been on that Pacific Whale Foundation trip a year ago picks up the tale. ''We spotted a humpback just about where we are now,'' she adds, ''and it spotted us too; it even came over for a closer look. Just off our starboard side, it lifted its eyes out of the water in a maneuver called 'spy hopping' because it allows the whale to observe surface activity. Then, when it had seen all it wanted to see, it dove right under us and seemed to carry us on its back playfully, as if we were its bathtub toy.''

They were a little frightened at the time, but about 20 of them are back this year to see if it will happen again.

During the commentary, there are intermittent interruptions: ''Four o'clock!'' someone yells. There is another shout for 11, and one for 7. And each time, everyone on the boat turns himself and his camera immediately in the right direction.

The seas we are sailing in this Sunday are a little rough and in color reflect the clusters of soft, leaden clouds that seem to rise from the west Maui Mountains and provide a constant, impenetrable ceiling for our expedition. It's a strange light in which to get a good picture, so Mr. Kaufman offers tips:

''All of you with cameras should meter them on the water,'' he says. ''Don't worry about the sky. You'd rather have a washed-out sky and a pretty whale than vice versa. If you have an automatic, take it off auto if you can and shoot manually, and shoot at the fastest speed possible; if you don't you'll miss the action. Whales may be big, but they move much faster than you think.''

The advice comes just in time. Off the starboard bow - 1 o'clock and very close - there is another pear-shaped blow . . . and another. Then two whales, one just seconds after the other, breach; they hurl their bodies clear of the ocean to crash back with a mighty splash. It's a whale version of the diver's belly-flop, and when 90 tons of whale hit the water it resounds like a cannon shot.

These are humpbacks, there's no doubt about it - 2/425ths of the humpback whale population in the North Pacific. Even though this year we don't play rubber ducky to a humpback, it's a very successful afternoon for whale watchers on the Aikane II. Practical information

The Aikane II is run by Seabird Cruises (120 Dickenson Street, Lahaina, Hawaii 96753). The trip I took cost $15 for adults for a 2 1/2-hour excursion; it is only one of the boats here that will take you whale watching. Another firm to contact is Kihei Sea Sports (811 Front Street, Lahaina, Hawaii 96753).

Whale watching in west Maui can be as close as your hotel. For instance, Kapalua Bay Hotel, where I stayed, has a catamaran of its own. It's somewhat smaller than the Aikane II, but it will hold 15 to 20 people comfortably, and on an hour-long sail we saw another humpback, two pilots, and dolphins.

Maui is a quick, half-hour flight via Aloha Airlines from Honolulu. Any number of airlines fly to Honolulu from the West Coast. Continental, for instance, has a daily flight out of Los Angeles, as do American, United, Western , and Northwest Orient. For price and other information on flights, the Kapalua Bay Hotel, and condominiums, contact your travel agent. For information on whale watching in the Maui area, contact the Pacific Whale Foundation; Azeka Place, Suite 303; Kihei, Hawaii 96753.