If you go dig into the story, he and his buddy TIED OFF THEIR BOAT to the WARNING BOUY above a massive "building sized" structure underwater (his words) and decided to go swim down to it and jack around.
The "building sized" structure (the largest one is 70' across and that's the one he willingly swam into) and 16' diameter pipe running from the structure in-land, was between the open ocean floor off-shore and a holding pond that was below sea level, directly on-shore and visible at a nuclear power plant, from where they moored the boat to the bouy.
Waaaaaaaay at the other end of the holding pond were the pumps for the plant. Nowhere near this thing. The pond is for cooling, filtration, and allowing crud to settle out, as well as a home to fish and other marine wildlife that swim through the pipe and hang out in there. The reservoir works by gravity and pressure. The intakes are off shore a ways to get deeper and cooler water.
He had to swim under support columns 6" high and 20-25' into the structure toward a 16" hole, and get close enough to it to get caught in the current. He's pretty much just a poorly trained, SCUBA diving, idiot.
http://imgur.com/a/Ve4to - an overhead photo showing how massive they are, and diagrams of the intake system. You can even see the warning bouy on the surface in the aerial/satellite shot.
People from the Internet have lamented that he was able to "get inside the plant" by swimming through the pipe. Others who live in the area point out that Ocean Drive and a bridge with a public road, cross the holding pond, and you could get out of a car and walk down and jump in if you like. You're not truly "inside the plant" there.
As someone else put it in a non-aviation forum... "Divers know you don't find random yellow warning bouys [anchored over "building sized" structures] in the open ocean."