You have to call. I think the number is 407-WDW-PLAY but I am not totally sure. If you want the Grand 1 make sure you call exactly 180 days in advance to book it. If you want floral or food brought on board the reservations agent can give you the proper numbers to call. If you just want a pontoon boat I don't think you need to give them as much notice as there are 8 of them... but I would still call 180 days out if you know the date you want.
The only problems we had were not being able to get the grand 1. We were told we had it, then we were told that we didn't and it was a mess. I have heard other people have had issues getting in touch with private dining but it always seems to work out in the end.