I booked the IllumiNations cruise (our first time) for late April. The CM told me that the boat would depart about a half hour before IllumiNations is to start (around 8:30 if IN is at 9:00), but you have to be at the dock no later than 45 minutes before (so in this example, 8:15). I was told the same when I considered booking a Wishes cruise, before I knew if I was going to be one of the lucky 4 to secure an IN cruise for that night!
I hope this helps - I'm really looking forward to trying this for the first time. It's a surprise for my husband's birthday!