It takes about 15 minutes or so and it is direct. Personally, when we are in a hurry or it looks like we just missed the boat, we've found the bus to be quicker and you will usually have it to yourself. Since this is probably your first visit, definitely take the boat at least once.
There are two boats. one goes direct to MK, the other goes to Fort Wilderness, then the Contemporary, then returns back to WL. Its a short ride, but a nice ride. There were times when we were the only ones on the boat!!!
Take the boat even if you just missed there will be another one alomng within 5 - 10 mins. When we stayed at WL we always took the boat over to MK. The bus stops at either poly or GF after it leaves WL. Plus we loved the boat ride.
The first time we stayed at WL we caught the boat to and from MK until the last day. Then we just missed the boat so went for a bus. It went straight there and took less than five minutes, how we wish we had tried it earlier. Since then we have used the bus much more. We did enjoy the boat ride when we had plenty of time but sometimes there was a wait for it to arrive, whereas the buses are very frequent and direct and never busy.