Went April 13, 2008....No problem. Just went May 19th, 2009...No problem.
My friend went April 26th, 2009 and they tendered ashore, but all water
activities were cancelled for the day due to rough seas.
Like Trip said, with Grand Cayman you just don't know from week to week. Chances are in your favor of everything being fine. We went there about 6 times in a row with no problems--then two years in a row we couldn't stop because of rough water.But as said, the odds are that you can make it.