June, when the sea is calm and already warm
June is the month with the fewest rough days for a boat tour on this coast. Off Monopoli the waves stay under half a metre for about 72% of the month and reach a metre or more only about 7% of the time, the lowest share of any month, so cave entries and swim stops are rarely lost to the sea. The sea at Bari averages 23.4 °C, comfortable for a swim stop for most people. Afternoons average 28.3 °C, with about 3 rain days, and the sun sets at about 20:27, which makes the sunset cruise a late one. The main drawback is a real one: it is already summer, so book a week or two ahead for the trips with the most reviews.
July and August, for the warmest sea
The sea reaches about 27 °C in July and August, the warmest of the year, and rough water off Monopoli is rare, about 11% of the time in July and 9% in August. These are the months for anyone whose priority on a boat tour is swimming and snorkelling. They are also the hottest, with afternoons around 30 °C and about 15 days a month at 30 °C or more, and they are the peak weeks for visitors to the coast, so a morning departure and an early booking matter more here than in any other month.
September, a warm sea and fewer visitors than August
September keeps a sea of about 25.5 °C, warmer than June, with afternoons around 26 °C. The drawback is that rougher days return: waves of a metre or more about 16% of the time off Monopoli, against 7% in June, so a booking for early in your stay gives you room to move it.
May and October, quieter and cooler
Both shoulder months have far fewer visitors than the summer, most tours still running, and afternoons in the low twenties. The sea is the difference between them: about 18.5 °C in May, a brisk swim at best, against about 21.5 °C in October, when a swim stop is still reasonable on a warm day. May has the calmer water of the two; October brings more rain days and more rough days as the autumn winds start.
November to March, limited and weather-dependent
Most shared tours have finished for the year. A few Polignano operators run into November, and after that what is left is private boats and cave trips on request. On a calm winter day the cliffs and caves are as good as ever with almost nobody else there, but the sea falls from about 18 °C in November to about 12 °C by February, so there is no swim stop, and it is rough far more often: waves of a metre or more between about 24% and 44% of the time from November to February off Monopoli. Ask an operator before you fix a date, and expect the skipper to make the call on the morning.