TULUM, WITH ITS long stretch of blindingly white beach, has long been the Yucatán's choicest escape. But change is on the way. On the narrow road along the beach, property-rights disputes with the ejidos have kept the resorts small along the southern section, since nobody can afford to pour millions into untitled land. But to the north (on the way to the famous Maya ruins), where property rights are more defined, at least one larger-scale resort is rising on the beach, and a sign nearby announces plans for THE TIDES TULUM, a Riviera Maya version of the Kor Hotel Group's five-star condo resort in Miami Beach. It's hard to say how long Tulum's barefoot spirit will hold out.

South of town, Highway 307 turns away from the coast, blocked by the great forests of the 1.3-million-acre Sian Ka'an Biosphere Reserve, and narrows to a single lane in each direction.

At the end of a deserted road, I discover a gorgeous beach and five perfect beach huts: "cabañas ecoturisticas," a faded sign reads. "bienvenidos." Xcalak is truly the end of the road—there's no bridge to Belize—and it feels like a castaway parish, remote and gloriously unpretentious.

Gas stations stretch thin across the land. English speakers here are forced to dig into their high school Spanish. Aside from the agricultural town of Felipe Carrillo Puerto, the road runs long and lonely through a verdant tropical forest broken now and then by tin-roofed shacks and children selling fresh papaya slices. About 40 miles north of the Belize border, a traveler faces a choice: Continue past beautiful Lake Bacalar, known as the Lake of Seven Colors for its shades of blue, to the state capital, the uninspiring border town of Chetumal? Or cut over to the coast, where the Yucatán withers down to a thin finger of peninsula?

I choose the coast road. Around lunchtime I roll into Majahual, formerly a picturesque fishing village. I say "formerly" because four years ago the big cruise ships began calling at Majahual (it's known as "Puerto Costa Maya" in the brochures), changing the town forever. Today its packed-sand main street bustles with massage kiosks, daiquiri bars, and Maya handicraft bazaars—all for day-tripping cruise passengers, who numbered half a million last year. Majahual is eager and ready to become the next southern outpost in the Riviera Maya. An entirely new section of town adjacent to the cruise port has been bulldozed and platted. Streets await concrete; lots await homes.

I continue south and realize I'm running out of coastline. Only 35 miles stand between me and Boca Bacalar Chico, the narrow canal cut by the ancient Maya that separates the Yucatán from Belize's Ambergris Caye. My destination is Xcalak (ISH-ka-lock), the last town on the map. It used to take half a day to drive down the molar-rattling beach road from Majahual, but in 1998 the government put in a paved highway that cut travel time down to an hour.

Francisco Suárez, an Amansala Resort employee, after spearfishing. (João Canziani)