What matters here is the swell at the mouth of the bay: with a ground swell the boat does not enter the Blue Cave and you swim outside, and in adverse weather the operator cancels and refunds. Check this before choosing your time.
Overcast clouds
Indicative reference based on OpenWeatherMap data for Kotor, at the head of the bay; at the Blue Cave, at the mouth, there is usually more sea. Whether to enter the cave is always the skipper’s call.