It used to only be offered as a Grand Gathering thing, but now you can book with smaller parties.
I have a bunch of friends that went a couple of years ago - they said it was fun.
I know a family that is going on it in the next week or so, too, so we'll see what they have to say.
I would guess the boat leaves the dock maybe an hour or so before Wishes? That's how the IllumiNations cruises go.