We always go in August. The first week of our trip the parks are generally pretty busy. I don't think I ever remember waiting more than perhaps 2 or 3 minutes for the Gran Fiesta Tour.
It is a nice, cool, quiet little boat ride, a lot like Small World. The kids love it, I love it. It is one of those "hidden gems" of the World Showcase. I think a LOT of people just don't realize it is there.