November is a transition month on the south coast and it rewards the travellers who understand what that means. The southwest monsoon is winding down through the first half of November. Showers are still possible, especially in the first two weeks, and the sea can be rough on exposed beaches. By the second half of the month, the coast typically dries out, the sky clears, and the dry season begins in earnest. What this means in practice is that November is cheaper and quieter than December but requires a little more flexibility about the weather.
The sea in early November is still unsettled enough that whale watching boats are not operating reliably. Most reputable operators in Mirissa begin running tours from mid to late November, and the season builds through December and into the peak months of January, February, and March. If whale watching is the main reason you are coming, mid-November onward is the earliest sensible window. Coming in the first week of November and expecting the whale watching season to be fully underway will likely disappoint.
For surfing, November is a good month. The swell from the departing monsoon produces clean waves at the main breaks, and the offshore winds that give the best surfing conditions on the south coast are well established by this point. Weligama, Ahangama, and Hiriketiya all surf well in November. The water is warm and the lineups are not yet crowded with the December influx of visiting surfers.
Accommodation prices in November are noticeably lower than December. The same guesthouse room that costs significantly more over Christmas is available in November at off-season or shoulder-season rates. Most places are open and operating normally by mid-November. Some smaller guesthouses and cafes that close fully during the monsoon reopen in early to mid-November, though a small number wait until December.
November is also one of the best months to visit Galle. The fort is quieter than it will be for the next four months, the restaurants are open, and the light in the late afternoon along the ramparts is exceptional. The Galle Literary Festival, one of the better cultural events on the south coast, typically runs in late January or early February, so it is not a November draw, but the town itself in November has a pleasant unhurried atmosphere.
The honest version: if your travel dates fall in the first half of November, pack light rain gear, keep plans flexible, and be prepared for the possibility of a grey morning here and there. If your dates fall in the second half of November, you are likely to have genuinely good conditions and noticeably better value than the peak season months that follow.