The boat to MK starts running 1/2 before the park opens. Our experience has been that a long line will form for the first boat of the morning, but most people will fit on the first boat. A second boat was waiting behind the first boat when we were there in August. So if there were any people who didn't make the first boat, they could take the second boat, which was only about nine minutes behind the first one. We were on the second boat and we still got into the park before rope drop.
If you want to get to MK earlier than that you can take the bus. It is a five minute ride and could be a little faster. The bus is not as much fun, though, and if you get a slow loading
ECV, it might take a little longer to load and leave.