Yes, swimming in Mallorca in October is a high-fidelity "insider" favorite, as the Mediterranean Sea retains much of the heat accumulated over the summer. The average water temperature in October is approximately 21°C to 22°C (70°F to 72°F), which is considered comfortable for most swimmers. While the air temperature begins to cool slightly, often averaging around 23°C (73°F) during the day, the beaches are significantly less crowded than in August. The "High-Fidelity" best spots for an October dip are the sheltered coves or "calas" on the south and east coasts, such as Cala d'Or or Cala Mondragó, where the water stays calmer and warmer. However, be aware that October can bring occasional "Gota Fría" (heavy autumn rain) storms, so it is a high-fidelity best practice to check the local AEMET weather forecasts. Overall, the first half of October offers some of the most pleasant, refreshing, and peaceful swimming conditions of the entire year in the Balearic Islands.