Oshkosh, Wisconsin (CNN) — For the biggest flying water bomber in the world, this is the end.

That's what Wayne Coulson, owner of a seven-decade-old flying boat called Hawaii Mars, said Monday as we stood next to it in a tiny dinghy on Wisconsin's Lake Winnebago.

"Hawaii Mars has reached the end of its career," Coulson said. "It's now time to reinvent it into something different than what it is."

This week the red-and-white firefighting water bomber is starring in the world's biggest aviation celebration -- the annual Experimental Aircraft Association AirVenture air show at Oshkosh.

How much do people love this airplane?

Well, aviation enthusiasts are paying $125 a pop to board a little boat and venture a few hundred yards out in the lake where it's anchored. There, they get a tour of this aviation gem.

For me, the plane redefined my idea of super gigantic when I got a rare opportunity to climb through a hatch and stroll out onto the plane's wings.

Wingspan: 200 feet.

Hawaii Mars' size and talents have helped it become a kind of aviation superhero, supported by beautifully shot documentaries on cable TV's Smithsonian Channel and Speedvision.

The plane is one of six Martin JRM Mars troop transport/medevac seaplanes made for the U.S. Navy beginning in 1941.

In the 1950s they were decommissioned, sold to a Canadian group in British Columbia called Forest Industries Flying Tankers and refitted as firefighting water bombers.

In 2007, Coulson's company -- also based in B.C. -- bought the last two surviving planes: Philippine Mars and Hawaii Mars.

With a wingspan a little wider than a 747, the Martin Mars is the largest flying boat ever flown operationally. ("Operationally" means that the freaky-huge, eight-engine "Spruce Goose" that Howard Hughes flew for about a mile in 1947 doesn't count.)

The Hawaii Mars can scoop up 7,200 gallons of water in 32 seconds. It skims the water's surface at about 80 mph, extends two 16-inch diameter scoops and fills up its tanks to prep for a water bombing drop. Inside the plane, the water is automatically mixed with a polymer thermo gel that sticks to burning trees and increases the dump's effectiveness.

When Hawaii Mars dumps that load, it can snuff out a four-acre wildfire in one fell swoop.

Pulling the trigger and dumping the load

Since 2009, Dev Salkeld has pulled the trigger on the Hawaii Mars somewhere between 600 and 700 times.

He's one of the pilots who's tasked with hitting a four-acre flaming bull's-eye with 7,200 gallons of water.

You might think the airplane includes some kind of sophisticated technology that helps Salkeld hit the target.

But you'd be wrong.

No technology here. Actually, Salkeld and his colleagues pretty much eyeball it.

"It's definitely hand/eye coordination," said Salkeld, who grew up in Saskatchewan. "Some people are better at it than other people."

The secret, he said, is doing it the same way every time, by approaching the forest fire target at the same speed, rate of descent or altitude. The problem is, "that's not always possible," he said.

The trigger for dumping the plane's 7,200 gallons is two switches located on each pilot's control yokes.

As the pilot in the left seat, Salkeld is in charge of flipping the switches. But if something goes wrong, the pilot in the right seat can also flip a switch on his yoke to dump the load.

"And if that doesn't work, you've got a big lever between the pilots that you can pull up and do it mechanically," Salkeld said.

Is it gratifying having a job where you're saving lives and property from devastating wildfires?

Apparently a man of few words, Salkeld would only say, "It's rewarding in that way."

Very selective

Coulson's water bomber has had trouble getting work lately, he acknowledged.

Neither the governments of British Columbia nor Alberta offered the Coulson Group firefighting contracts during the recent wildfire season, prompting the decision to put both Hawaii Mars and Philippine Mars on the market.

The asking price is $3 million for each.

"We're going to be very selective on where these airplanes go. They're aviation royalty," said Coulson, whose company has been working with the National Naval Aviation Museum in Pensacola, Florida, on the trade of the Philippine Mars.

But there are some snags that need smoothing for that deal to come through.

And as the Martin Mars water bombers begin to exit the world stage, China seems prepared to fill the gap with what it's touting as the "world's largest seaplane," the amphibious AG600.

The first plane was unveiled in China over the weekend.

The retiring Martin Mars aircraft will be missed.

After working aboard Hawaii Mars for 18 years, flight engineer Glenn Pley knows his days on the flying boat will end soon.