THE RIVER IS AS BLACK AND FLAT as freshly screeded tar. Submarine visibility is at a distressing half-inch or so, max. Anything could be down there.

I'm floating the Wekiva River north of Orlando, moving at what feels to me like a pretty good clip, when a husky teenager in a decrepit red kayak slips up alongside.

"Excuse me," I say. "Are there alligators in here?"

"Yeah, kind of," he replies. "Just saw one back there. Not too big. Seven feet, about."

"Back where, exactly?"

Obviously more intrigued by my vessel, he halfheartedly jerks a thumb upriver.

"Where'd you get that boat?"

"I made it," I say.

He hoists his eyebrows in an expression of very mild impressedness.

"Awesome."

He toodles off, leaving me to fend for myself in a craft that probably wouldn't survive an attack by a determined koi. I'm piloting an inner tube, and I'm doing so with great pride. I'm confident that, gator vulnerabilities aside, mine is the swankiest custom-made personal flotation device ever to cruise a backwoods creek. The chassis (courtesy of Ontario, Canada's Tube Pro) is a 44-inch tire tube sheathed in a ballistic-nylon skirt of bold electric blue. It was already a handsome little barge, but with a long trip ahead of me, I spent several days in the shop of my fabricator friend George souping it up with post-factory snazz. With calipers, compasses, and a Delta upright bandsaw, we cut a 20-inch birch-ply deck to support a seat back and armrests (butchered from a lawn chair) and an extremely bitching adjustable sun canopy. Then, with great effort and cursing, I hammered brass grommets into the nylon skirt, through which I secured the deck and lawn-chair anatomy with zip ties. Next, we stitched a CAUTION: SMALL CRAFT! pennant out of orange camo fabric and, amid a shower of sparks, chopped some threaded rod into three two-foot lengths, creating a portable flagpole I could reassemble with couplers. Plans for a sail and rudder foundered at the blueprint stage. To my greater sorrow, schemes for a wet-bar-and-cooler sidecar never made it out of the wind tunnel.

I built this glorious rig not merely to astound the locals but to keep me comfy on what was supposed to be a five-day river voyage through an intermittent fantasy river, made up of my own quixotic selection of the crystalline, spring-fed waterways of north-central Florida.

The basic concept is a loaner from John Cheever's classic short story "The Swimmer," from 1964: One hungover Sunday, Neddy Merrill and his wife are drinking gin and lounging poolside at the Westerhazys' when Neddy gets the idea to swim the eight miles home, through a suburban "river" composed of the innumerable backyard pools strewn across New York's Westchester County.

"His life was not confining," writes Cheever, "and the delight he took in this observation could not be explained by its suggestion of escape. He seemed to see, with a cartographer's eye, that string of swimming pools, that quasi-subterranean stream that curved across the county. He had made a discovery, a contribution to modern geography; he would name the stream Lucinda after his wife. He was not a practical joker nor was he a fool but he was determinedly original and had a vague and modest idea of himself as a legendary figure. The day was beautiful and it seemed to him that a long swim might enlarge and celebrate its beauty."

My River of the Mind, however, is a far larger and even more harebrained waterway than Neddy's chimerical stream. As I've conceived it, mine will loosely trace the artesian output of the Floridan Aquifer, a subterranean freshwater sea whose natural eruptions—primarily in the northern half of the state—pay out some of the purest water, flowing through one of the most staggeringly beautiful riparian corridors, in the world.

I've waited for the more congenial temperatures of September to tube an ambitious seven springs and rivers, starting here in suburban Orlando, then north toward Gainesville, and then south toward whatever river will spit me out into the Gulf of Mexico. I haven't figured that last part out yet.

Due to a few logistical miscalculations, the plan isn't coming together too well. First, I didn't mean to begin my journey in this Stygian creek. I'd meant to embark upriver, at Wekiwa Springs State Park, whose pure waters resemble nothing so much as a large, handsome spill of Crest Gel.

To my dismay, when I showed up this afternoon, a park employee informed me that any "craft" not approved by the Coast Guard was forbidden. He then directed me downriver—but not before remarking that deploying a tube in the Wekiva was not really something he'd recommend.