As someone said,Grand Cayman can be a hit or miss thing, regardless of the time of the year. The first 5-6 times we were there, we never had a problem. Then we went 2 years in a row and had to miss due to rough water. But, the last 2-3 times since then we've not had a problem. More likely than not, you will make the stop.
Be sure to visit the Tortuga Rum Cake factory and get some wonder rum cake to take home with you.