We visited this Manzanillo in December and you would have overheated if you didn’t have shorts and a T-shirt on. The days were partly cloudy, warm, and quite humid. Temperatures during the daytime were hovering in the 80’s. One word of caution is that it is very easy to get sun burnt even if you are not exposed to the sun for very long.
It is especially easy for sun starved tourists who are vacationing from much colder climates to lay out in the sun for several hours. June through September tends to be extremely humid and rainy.