My experience is that it is no more likely to be raining the week of your cruise than on any other week during the summer. Pop-up showers are a fact of life in the Caribbean in the summer. But they rarely last long. Obviously if a tropical wave or hurricane develops, things will change. But then again, if a hurricane is in the western Caribbean your ship will not be there. One thing I can tell you is that Cozumel in August is very hot and humid.