File photo

It was clear right away it was a tuna swimming past because of the distinctive bumps along its back that lead to its angular tail. And what a whopper. It was about the size of a small person, and seemed unbothered by me bobbing just a few feet away.

It was my second dive, with a colleague, in the remote Marshall Islands, located midway between Hawaii and Australia. There's little tourism here, but plenty to see below the surface. During our first dive, in the Majuro lagoon, we'd explored a sunken plane, helicopter and ship in water that was warm and crystal clear.

We started with the plane, an old DC3 that was resting on the sandy bottom at a shallow depth of between 3 and 6 meters, making it accessible for novice scuba divers and even experienced snorkelers.

Fish darted about what were once the cockpit controls and we could see an old strap still hanging inside a window. Along the side we could make out most of the lettering: Sea Star Pacific.

A little deeper, the helicopter was shrouded in seaweed and tube-shaped growths. Deeper still was the ship, its rooms and decks accessible with careful maneuvering and the aid of an underwater flashlight.

I thought at first we might have entered some kind of Bermuda Triangle where craft regularly come to grief. But our guide, Hiroaki Ueda, explained the wrecks had been towed there for divers to enjoy.

Ueda first came to the Marshall Islands from Japan in 2007. The dive outfit that hired him soon went bankrupt and so he opened his own business, Raycrew. He's completed some 4,000 dives all over the islands, he said, and loves the endless color and life he finds in the coral.

But climate change is having an impact throughout the Marshall Islands, which are vulnerable to rising seas and storm surges. Last year, Ueda said, he saw extensive coral bleaching, which is when warmer water temperatures cause coral to turn white, increasing its risk of dying.

He took us outside the lagoon to the oceanside coral for our second dive. This time we went deeper and I encountered the tuna at about 30 meters. We swam alongside a coral bank that dropped away steeply into the abyss below.

We saw a turtle swimming lazily along and swam next to it for a short distance before it seemed to sense it was being cornered and darted away.

Ueda runs his business from the Marshall Islands Resort, one of just two Western-style hotels in the capital, Majuro. Life on the islands is slow-paced, and sometimes things like the Internet, or the electricity, don't work. In populated areas, the lagoon is ringed with trash, although some young activists are organizing drives to clean it up.