While you can technically swim in Majorca in December, most people would find the water temperatures quite bracing. The Mediterranean Sea around the Balearic Islands drops to approximately 15°C to 17°C (59°F to 63°F) in December. For context, this is significantly colder than a standard heated swimming pool (usually 26°C) and is more akin to a "cold plunge" experience. While you will see some hardy locals and "winter swimmers" in the water, most tourists prefer to stick to the sand for sunbathing on sunny days. If you are determined to swim, a 3mm wetsuit is highly recommended for comfort. The air temperature in December is often a pleasant 16°C to 18°C, making it perfect for hiking or cycling, but the "swimming season" is officially over. If swimming is a priority for your December trip, it is best to book a hotel with an indoor heated pool or a "spa" circuit, as the sea will likely be too cold for a relaxing, leisurely swim for the average vacationer.