Adriatic sea: around 23-24 °C, except alomng most of the Croatian coast where temperatures are down to 21-23 °C (locally in the Kvarner bay even down to 19 °C). Ionian and central Mediterranean sea: is around 23-25 °C.

Generally, the Mediterranean Sea as a whole is warmer than the Adriatic Sea, especially during the early summer and autumn months. The Mediterranean is a vast, deep body of water that retains heat well, particularly in its southern and eastern basins near Greece, Cyprus, and North Africa. The Adriatic, being an arm of the Mediterranean that extends north toward the Alps, is significantly shallower, especially in its northern parts near Venice and Istria. Because it is shallower, the Adriatic heats up very quickly in the peak of summer (July and August), often reaching temperatures of 25°C to 28°C, which can feel warmer than the open Mediterranean. However, it also cools down much faster once the "Bora" winds begin to blow in the fall. In 2026, climate data shows that both seas are warming at record rates, but for a guaranteed "warm" swim in late May or October, the southern Mediterranean remains the more reliable choice for travelers seeking balmy water temperatures.