Discovery Cove only allows a maximum number of guests per day (1300), so you never have to worry about it being crowded regardless of what day you pick.
With that in mind, we have always selected our DC visit days based on when we thought the other theme parks would be busiest, knowing a day at DC would guarantee us no crowds on an otherwise crowded day at Disney, SW or Universal.
So I would say check the crowd calendars for the other parks of your visit, and pick DC for one of the dates that is projected to be the busiest.