He grew up around airplanes. His father set a world record in a B-58 Hustler back in the 1960s.

Denver had quite a bit of flying experience but only a few hours in a Longeze. Not only that, this particular Longeze had a modified fuel system that effectively set Denver up for the accident. The normal Longeze fuel system has the fuel tank selector switch up front between the pilot's legs where it's easily accessible.

Some pilots get paranoid about fuel in the cockpit because of the chances of a post-crash fire. The builder of Denver's Longeze fell into this category. Instead of routing the fuel lines to the normal selector switch up front, the builder put the switch in the rear of the plane. To switch tanks, the pilot had to release the control stick (mounted on the right side of the cockpit like an F-16 so it'd be hard to operate with your left hand) and reach across with a pair of pliers and rotate a rod over his left shoulder. Here's a summary of the NTSB accident report that verifies what I'm saying.

Denver had landed shortly before his last flight. Being unfamiliar with the plane, he misread or misunderstood how much fuel was left in the tanks so he didn't take on any more gas. Shortly after takeoff when the plane was only about 200 feet high, the engine cut out due to fuel starvation. Denver was apparently trying to switch tanks when he hit the water.

Had he been more familiar with the plane, he probably would've taken on more fuel. Had the plane been built according to plans, switching tanks would've been easy and safe. As it was, it was literally an accident waiting to happen. The Longeze was designed by Burt Rutan and is a strong, safe plane. However, like any homebuilt, it has to be built and operated properly.