In America, I've watched with delight as humpback whales return on their migration from the Caribbean to Cape Cod. once there, they feed greedily on sand eels using an amazing technique known as 'bubble-netting' - blowing a stream of bubbles as a net around their prey.

One afternoon, our boat was surrounded by 75 whales engaged in this, one of nature's greatest spectacles. I've seen humpbacks breach only feet away, launching their massive bodies into the air like vast angels, with their 15-foot flippers as wings.

I've seen the world's most endangered whale, the North Atlantic right whale - of which fewer than 400 individuals remain - feeding just a few feet off the New England shore. off Kaikoura, New Zealand, I swam with 300 dusky dolphins zooming around me.

And, thanks to the Department of Conservation there, I encountered one of the world's rarest cetaceans, the tiny and adorable Hector's dolphin. For some reason, it is fond of humans singing to it - which led to the silly sight of this puny Englishman floundering in the Pacific while yodelling to a dolphin.

But, most memorably of all, I've swum with the ocean's mightiest predator, the sperm whale - the model for Herman Melville's moby Dick.

For centuries, this species was hunted for the oil in its huge, square head. Known as spermaceti, it burned clear and bright; lighting city streets and houses from London to Paris.

Yet this ancient animal evolved millions of years before man. It can dive for more than a mile and stay below for two hours in search of the giant squid it feeds on. It also has the largest brain of any creature and communicates in a series of morse-code-like clicks.

Some scientists, such as Dr Hal Whitehead, of Dalhousie University, Nova Scotia, even believe that sperm whales are so self-aware that they might have begun to evolve a concept of religion.

That may sound absurd, but having dived in the three-mile-deep waters of the Azores with these truly placid, gigantic creatures, I can bear personal testament to their intelligence. Swimming towards a pod of 14 sperm whales, one large female detached itself from the group and swam straight towards me.

I've never been so terrified. I could even feel the click of the whale's sonar as it scanned me, physically reading me. Then it came close enough to touch, and turned its eye to look at me. In that gaze, I saw sentience. And all I could think was of one word: sorry.

Sorry for treating you and your kind with cruelty for man's selfish gain. A soft-hearted notion? I don't think so. Scientists are increasingly sure that cetaceans are highly intelligent animals.

Dolphins can recognize themselves, not only in the mirror but also on televisions pressed against the glass wall of underwater tanks - an ability which places them above the great apes in intelligence.

There is good reason to suppose that their cousins, the great whales, share this intelligence.

We just don't know, because it is so hard to observe these creatures. They live in an environment which is as alien to us as outer space. We have a better idea about the surface of the moon than we do about the deep ocean.

Amazingly, there are species of large whales, known as beaked whales, which have never actually been seen alive - only their skeletons are testimony to their existence.

When I was in New Zealand, Anton van Helden, the curator of marine mammals in the national museum Te Papa, showed me the skull of a spade-toothed beaked whale. It was one of only three known specimens in the world.

No accurate illustrations, let alone photographs, exist of this animal. It has never been seen alive. These are the wonders of the natural world that deserve our protection.

Yet the threats faced by whales and dolphins make them the barometers of our world - and even amid the vast ocean depths in which they live, their habitat is in jeopardy.

Research has shown that sperm whales are being contaminated by chromium, which these deep-breathing whales inhale from chemical plants they pass on their migratory routes.

It is believed the chemical is causing damage to the whales' calves analogous to Down's syndrome. meanwhile, a fragile population of 1,700 sperm whales is faced with the appalling prospect of ingesting crude oil from the leaking well in the Gulf of Mexico.

Worldwide, the noise pollution we create in the oceans from shipping and military sonar is causing serious problems for animals which live by their sense of sound. Global warming and acidifying oceans affect their food resources, forcing them to travel farther and farther in order to feed.

Can we seriously say, as the Japanese, Norwegians and Icelanders do, that these animals are robust enough to be hunted again? They claim we sentimentalize these creatures.

After all, they say, we kill cows and pigs - what's the difference? Well, those are domestic animals. They are also slaughtered humanely. It is impossible to kill a whale humanely on the high seas.

It is also immoral, if you accept that these are sentient creatures - capable of grief, complex social lives and a language of their own. Perhaps, in years to come, we'll even learn how to interpret whale speak.

I'm not sure I want to be around if we do. I don't think I could bear to hear what they have to say.