While tourists and commuters cruised effortlessly above him, Rod Moore explored Sydney Harbour the hard way, by walking along its bottom.

And if the surface is a generally smooth and glistening vision for anyone in a boat, below is a murky and sandstone boulder-studded slog, according to the underwater walker.

Underwater walker Rod Moore emerges from Rushcutters Bay at the end of his trek across Sydney Harbour.

"It was like mountain climbing at times," said Mr Moore, catching his breath after the six-hour walk. "The rocks were up to 10 feet high and covered in kelp."

Having walked three kilometres under Lake Macquarie last year, Mr Moore thought he would trek along the bottom of the harbour. But this journey, he said, turned out to be almost twice as long, more than double the depth, and immeasurably harder than the lake walk.