when we stayed at WL 2 years ago and had 8:00 PS at CRT, we had to take the bus as the boat was not going to be starting pick up until 30 minutes before park opening (9:00 on this particular day). I do believe the bus stopped at TTC, but we did not have to get off and transfer to another bus - it just made a quick stop and then proceded to drive to MK. (it might have stopped at CR on the way also, I can't quite remember) I don't think the trip was too long - maybe 15 minutes?
I would recommend getting to the boat dock about 10 minutes before they are scheduled to pick up so if it's crowded, you can be sure to get on the first boat that comes. I think they run every 20 minutes or so - there are signs that indicate how often the boats arrive/depart and where they are going to. Also, when you do take the boat over to MK, the ride only lasts 5 - 10 minutes and it's a nice relaxing way to travel!