We were sailing off the island of Hunga - part of the Vava'u group - when we saw two whales coming towards us and I decided to jump in and see what they looked like in the water.
I swam a little way from the boat into their path and kept looking down into the blue void but saw nothing. All of a sudden two enormous shapes materialised, coming straight at me. I was terrified, my heart was thumping and I couldn't move. But they just glided down underneath me and slid past, big eyes looking up as they went by.
They were huge. Like a 747. The pectoral fins were like wings, 5m long on either side, plus the width of the body, so that's 15m wide in all - the length of the body even longer.
When you see them on the surface it's like an iceberg. It's just the tip. Most of the body is underwater and it is massive.
Afterwards I climbed into the boat and I was dumbfounded. I couldn't find the words to explain to my friends on the boat what I had just witnessed. I couldn't describe the scale of what I had seen in a way that they could understand.
Unfortunately that's not my experience but that of Allan Bowe, the New Zealander who pioneered swimming with whales in Tonga, talking about his first encounter with humpback whales 15 years ago.
It's an experience I'd really like to have. In fact, three years ago I wrote a column about 12 things I'd like to do before I die and swimming with whales was one of them.
So I was very excited when I got the chance to go out with Bowe's Whale Watch Vava'u during the August-September period when the humpbacks travel from the icy food-rich oceans around Antarctica to the warm waters of Tonga to give birth.
But, while we saw one lonely male and heard the sound of his singing through the hull, then watched entranced as two adults - probably a male and a female - played around the boat, rolling their huge bodies over, slapping fins and tails and generally showing off, conditions weren't right for me to get into the water.
That was disappointing, naturally, but hardly a surprise, because the whales are wild and free, not circus animals that perform on command. And it's impossible to control the weather. Bowe reckons you really need to allow 3-5 days to be reasonably sure of being able to swim with the humpbacks and I only had one.
But, after talking with people who have swum with the whales, I'm more determined than ever to go back and do it. It's obviously a deeply moving experience.
Bowe, who has taken hundreds of people out whale watching, says almost everyone who swims with these huge, gentle creatures is emotionally affected.
"Two years ago I took a lady of 84 into the water, I guided her myself and held her hand. When we got her back on board she cried for at least 10 minutes, she was emotionally overcome. It's quite common for girls to cry when back on board.
"Mostly the guys will sit in a stunned silence, not talking to anyone, just sitting gazing blankly into space, trying to get their heads around what they've just seen ... which is pretty much what happened to me the first time."
Bowe found his own first experience so amazing that he tried to persuade his friends on the boat into it as well. "I managed to talk a couple of them into getting into the water but not all would. Some weren't brave enough, or silly enough, depending on how you look at it."
But the catalyst that led him to develop that experience into a commercial venture came from a meeting with another Kiwi, Barbara Todd, who pioneered whale watching in Kaikoura. "She came on to my boat up here one night, we had dinner and talked about it, and she sowed the seed in my mind."
The first step, Bowe says, was to go to the Tongan Government and apply for a whale watching licence. "They thought I was mad. And of course they had no precedent to go by. But they agreed and the first licence cost me $40."
These days, of course, no one thinks he is mad. There are now 13 whale watching licences, with two boats per licence, and the experience is a major tourist attraction for Tonga. In fact Bowe has been honoured for his service to the country.
Over the years he has also developed a four-fale luxury eco-resort on 6.5ha Mounu Island - "Getting the lease of the island cost me one pig and two fish" - which is 30 minutes by boat from the Vava'u capital of Neiafu and a perfect base for anyone wanting to go whale watching.
Bowe is well aware of criticisms that his tourist venture is bad for the whales but he maintains that is based on ignorance of the reality. "Most of the critics haven't even come to see what we do.
"When we approach them we always leave the motor running so they know where we are, we put people in the water 20-30m away, no more than four at a time with a guide, then they swim gently forward and let things develop."
"We're certainly not harassing the whales. It's not in our interest to harass them because if they get fed up we haven't got a business.
"If the whales are interested they will often come over but if they're not, one gentle sweep of that huge tail and they're gone."
On the contrary, Bowe says, whales often give the impression of enjoying the interaction and they certainly seem to go out of the way to avoid hurting people. "I had a client in the water today with two adults, and at one stage both were lying vertical with their tails right out of the water, maybe 5m up in the air. As one moved away it dropped its pectoral fin and could easily have collected him but it clearly made sure it didn't. They're very conscious that you're there in the water and not once have we seen any sign of aggression. They're gentle giants."
Swimmers do have to be more careful with the calves because "like any young animals they're very inquisitive and playful but they weigh 1.5 tonnes and they can give you a bump.
"I push them away if they get too close - the skin feels like a dive suit only quite warm - but they have been known to bruise people."
The fact that mothers will let their calves approach swimmers, he says, is the clearest indication that the whales don't feel harassed or threatened in any way. "A mother usually starts off by keeping herself between swimmers and her calf but she almost always relaxes and lets it come up. If they felt any sense of threat they would never come near us."
* Jim Eagles visited Tonga with help from Air New Zealand and Tonga Visitors Bureau.
Air New Zealand flies to Tonga five times a week with fares starting from $193 one-way, plus airport charges and government taxes.
Airlines Tonga, operated by Air Fiji and Teta Tours, has regular flights to Vava'u. Ph (676) 26125 or email firstname.lastname@example.org.
Mounu Island is on the web at www.mounuisland.com.
Options in Neiafu include: Sovereign Residence, a boutique guesthouse and restaurant (email@example.com); the Paradise International Hotel (www.tongahost.com); and The Tongan Beach resort (www.thetongan.com).
Whale Watch Vava'u is on the web at www.whalewatchvavau.com.
See the Tonga Visitors Bureau website at www.tongaholiday.com or ring the Auckland office at (09) 629 0826.