Mosquito Bay, Vieques

Vieques is a small island off the east coast of Puerto Rico, known for wild horses, quiet beaches and Mosquito Bay, a bioluminescent bay often described as one of the brightest in the world. The glow comes from huge numbers of dinoflagellates that light up when disturbed, so a paddle or a swimming fish leaves a trail of blue light. The reputation is strong enough that many travelers choose Vieques over the mainland bay if they can spare a day. From San Juan, the easiest way is an organized day trip. Viator lists a Vieques Bio Bay and beach day trip from San Juan by ferry, with more than two hundred reviews. The listing describes hotel shuttle pickup in San Juan, a drive to the Ceiba ferry terminal with ferry tickets included, time at a Vieques beach, and a guided clear-bottom kayak tour of the bay before the return ferry. It takes about eleven hours.

The day is long but well organized. You leave San Juan in the morning, cross by ferry from Ceiba, spend the afternoon on a Vieques beach such as Sun Bay or Esperanza, and then paddle at dusk with a guide. Because the return ferry timing matters, the operator manages the schedule, which is the main reason to book a tour rather than to try this yourself. The cost reflects the long day, the ferry and the guide, so it sits well above a mainland kayak tour. If you want more time on the island or a quieter pace, consider staying overnight, which is outside the scope of the standard tour.

Conditions drive the experience. As with every bioluminescent bay, the dark of the new moon gives the strongest glow, and a bright full moon, heavy rain or rough water can dim it or cancel a tour. The ferry itself can be delayed by weather or demand, so choose a free-cancellation booking and keep your schedule flexible on the day. Bays are protected, and chemicals such as sunscreen and bug repellent harm the organisms that create the light, so follow your guide's rules about what to apply and whether you may enter the water.

Who it suits: couples, nature lovers and travelers with a day to spare who want the best bay. If your time is tight, Laguna Grande in Fajardo gives a similar effect with a shorter journey. Bring a swimsuit, water shoes, a dry bag, a light layer for the ferry and your ID. Puerto Rico is part of the United States, so US citizens do not need a passport, but carry government photo identification.