We will be doing Discovery Cove in October 2012. We are shooting for mid-October because we did a Disney water park this year at the END of October and it was kind of on the cool side.
October is when we are going for the lower crowds. But if you wanted more comfortable (hotter) weather then May would be the better choice. If you went during the first 2 weeks of May you might still find decent crowds because most schools don't start getting out for the year until the 3rd week of May and in most places the 4th week of May.