Everything in Bacalar happens on the Laguna de los Siete Colores, a freshwater lagoon forty kilometres long lying over white limestone, which is what produces the colours — the shallows read turquoise and the channels read something close to navy, and the line between them moves with the light.
A sailboat is the right way to see it. Engines are faster and worse; the standard circuit takes in the Cenote Negro, the narrow Canal de los Piratas and the Rápidos where the water runs. Pontoons work for groups, kayaks and paddleboards for a quiet early morning before the wind gets up — which it reliably does by early afternoon.
The lagoon's stromatolites are the thing to understand before you get in: mounded microbial reefs, structurally the same organisms that oxygenated the atmosphere billions of years ago, and still growing here in one of the few places on earth they survive in fresh water. They look like rocks. Standing on them kills them, so get in off a dock or a boat ladder, not off the shore, and keep sunscreen out of the water.
Bacalar is also the jumping-off point for the Chacchoben and Kohunlich ruins inland, and for the ferry route south to Belize.