I don't remember the exact time the boats start running, but I know when we had to leave for an early ADR they were not running yet.
Take the bus. The buses are very easy and fast that early in the morning.
Do NOT drive your car to the MK.....you have to park in the parking lot of the TTC and then take a ferry or the monorail to the MK.
It is much faster to use the bus for early ADR's in the park.
As far as how much time to allow, I'm not exactly sure....we got lucky and a bus pulled up right when we got to the bus stop. We were at the gates of the MK less than 20 minutes after walking out of our hotel room.
You should leave the WL no later than 7:15. Honestly, if it were me I would probably leave my resort room at 7:00 just in case you have to wait for the next bus. If you get there too early, you can take your time walking down a nearly empty Main Street. You can get some wonderful pictures.