Air temperature is what the brochure prints. Sea temperature is what decides whether you actually get in. So this ranking runs on the water: twenty eight beaches where January still delivers real heat, ordered by average January sea temperature, warmest first, with the air figure alongside in Celsius and Fahrenheit. The spread is wider than you would guess. At the top, Nungwi in Zanzibar and the lagoon at Bora Bora both hold around 29C (84F), which is warmer than most people run a bath. At the bottom, Playa de las Teresitas in Tenerife manages 20C (68F), cold enough to make you think about it and still the warmest beach in Europe in January. In between sit the Andaman coast, the Indian Ocean atolls, the Caribbean and the Brazilian Atlantic. Figures are January climate normals rather than forecasts, so treat them as what the month usually does, not as a promise about the week you book.
Nungwi, Zanzibar: Sea 29C (84F), Air 32C (90F)
Nungwi sits on the northern tip of Zanzibar, where the reef drops away close to shore and the tide does not strip the beach bare the way it does further south. That geography is why it tops this ranking. The Indian Ocean here holds around 29C (84F) through January, warmer than most heated hotel pools, and the air pushes past 32C (90F) by early afternoon. Wooden dhows work the shallows all day, their sails catching a light that turns the water an improbable turquoise. January falls inside the short dry window between the rains, so expect long clear mornings and the occasional brief afternoon shower. Flights from most European hubs route through Doha, Dubai or Nairobi and land in roughly eleven hours of air time. Book the north coast if you want to swim at any hour, not just at high tide.