As dawn breaks over Mexico City’s floating gardens, Omar Menchaca paddles his kayak through a maze of canals collecting garbage left by visitors to one of the last vestiges of the ancient Aztec capital. In the silence of the early morning, before the hordes of tourists arrive, the 66-year-old retiree fishes plastic bottles and other debris from the waters of Xochimilco. “I came here to train for my competitions,” says the former athletics champion. “Over time, unfortunately, I started noticing that these canals were full of garbage.” As his single-seater kayak glides by, herons and pelicans ta…