Seasonal thermocline: 10°C differences within a few metres. Phytoplankton increases with warmth. August: boat congestion, sediment resuspension. Sep-Oct: 20-25°C, thermocline fades, consistent visibility 15-30 m.
In July and August, the Mediterranean builds up pronounced thermoclines that fragment visibility, and plankton blooms can reduce it considerably in areas such as the Costa Brava or southern Valencia. Added to this is the overcrowding of boats and the saturation of dive centers. Autumn (September to November) emerges as the preferred season thanks to the combination of visibility, temperature, and lower visitor numbers.
The main culprit is the seasonal thermocline, a layer that can mark differences of up to 10 °C in just a few meters and distorts visibility. Phytoplankton blooms add to this, with their concentration increasing with heat and intense sunlight. August coincides with the peak of tourists and boats, generating greater sediment resuspension. In contrast, September and October offer an almost perfect balance: still warm water (20–25 °C), a diffuse thermocline, and consistently 15–30 meters of visibility.