A rare underwater world in Hawaii

It's pitch dark and I'm dangling from a rope beneath a boat drifting a couple kilometres offshore, like bait on the end of a hook.

Voluntarily, I might add.

In my nightmares, a 6m shark comes shooting out from the depths, grabs me by the leg and yanks me down. But so far tonight I've been lucky.

I'm one of a dozen scuba divers spending a week aboard the Kona Aggressor II. We've been puttering around the Big Island of Hawaii, logging up to five dives a day, and not doing much else except eating, sleeping and reading. For an avid diver, it is heaven.

Right here, right now, we're armed with dive lights and underwater cameras, peering into the darkness, waiting for whatever might well up from the depths to dine on plankton. What swoops up is startling: an array of invertebrates, mostly clear, others with flickering blue or green or red lights that look like futuristic spaceships or creatures from a sci-fi movie.

Some resemble fuzzy thimbles, others are long cones; some wave long, wispy tentacles, and a few flex and fold like a beating heart. They range in size from an inch or two to several feet. And now and then, a flying fish zooms past - something that's rarely seen by divers (at least not in the water) during the day.

After an hour, we shimmy up the rope and onto the back deck of the 80-foot boat, giddy with excitement. After a mug of hot chocolate to warm up, I head to bed.

I've done a lot of terrific diving this week, exploring the volcanic topography beneath the waves. We've swum into huge tubes created by cooling lava, drifted over underwater craters and peered through beautiful underwater arches.

For me, a live-aboard dive trip such as this is the way to see the ocean. But it's not for everyone. The boat is comfortable and tidy but hardly plush. If you don't like to dive - a lot - this is not the vacation for you. And it's not about fancy dining - although after 10 live-aboard dive trips in the Caribbean, the Galapagos Islands and Fiji, I've decided this boat serves up some of the best food. It's practically gourmet.

A few things stand out about diving in Hawaii. First, there's an eel in every rock and crevice. Second, the array of worm-like creatures called nudibranchs, which look like colourful bits of melted crayon oozing over rocks and corals, is dazzling. One, called a fried egg nudibranch, looks like a fat pale-blue finger topped with black smudges and yellow bumps.

More than 20 per cent of Hawaiian reef fish are found nowhere else in the world, and it's fun checking them off your life list. We've seen variations of butterfly fish and angelfish we've never seen before, even with more than 300 dives under my (weight) belt.

The Big Island of Hawaii is known for its manta rays, and one of the best dives here is just offshore from the airport, where the floodlights attract food that chums up the giant rays. We sat on the rocky bottom one night and watched for 45 minutes as a 14-footer and a pair of 10-footers flapped their enormous wings, making huge looping rolls as they scooped up the plankton in the water. Unbelievable.

It's not just what we've seen beneath the surface that mesmerises, either. One day we spotted a pod of a dozen or so rare pygmy killer whales off the bow of the boat. At first we thought they were dolphins, but their blunt foreheads and movements quickly clued us in. We watched for half an hour.

Another afternoon, while we were moored in Kailua Bay, a group of about 30 dolphins arched through the waves a few hundred yards from the boat. I jumped in with two crew members and swam toward them. At first, they zipped away, but eventually they let us get fairly close. For about 20 minutes, we swam above them, listening to them chirp and watching them circle. Now and then, a few would flick their tails and dart powerfully to the surface.

That convinced me. In my next life, I want to be a dolphin: graceful, magnificently strong and playful. That way, I'd get to spend more time exploring this underwater world that most people never see.

IF YOU GO

The Kona Aggressor II runs weeklong dive trips from Saturday to Saturday. Passengers embark at Kailua Pier in downtown Kailua-Kona on the Big Island of Hawaii. Cost is $2735 a person (watch for specials) and includes all meals and dives. (Tanks provided; bring your own gear.) For more information, see aggressor.com/kona.php. Flights are available into Kona International Airport.