They do run several boats and I've never seen it fill completely up with people aside from closing time at Epcot or MGM, even during Christmas/NYE week. If you're going over at lunch, you'll likely have your choice of seats inside or even on the bench outside in the back of the boat. We always sit there if it's pleasant outside.
Give yourself 45 minutes from the time you leave MGM's front gate to the time you want to be walking up to Le Cellier and you'll be fine taking the boat (the ride isn't more than 25-30 minutes, but that will give you extra time if you have to wait for a boat to arrive). As you leave MGM, the boat dock is to the far left. When you get off at Epcot, just head to the right and you'll see the International Gateway entrance. There are a few security tables set up there and it always moves quickly. Once inside, head left, walk past the UK and most of Canada. Just as the main path turns right toward Future World, you'll see the garden path entrance to Le Cellier.