The bus ride from MK to WL is super short, like 5 min. Or so. But, like all other bus servcice you could wait up to 20 min or so for the next bus, if you happen to have bad timing.
The boat is a nice relaxing ride, I want to say about 15 min, but don't quote me on that. Bus is definitely quicker, but the boat is the nicer ride,if you have the time.
I would recommend the boat. The boat ride is less than 15 minutes long and is very relaxing. Also, you get the opportunity to walk from the dock to the resort and take in the awesome landscaping and theming of this resort.
As for time, even though it is not a very long ride, I would recommend giving yourself 45 minutes from your ADR. It's not a big deal if you are early because IMHO, this resort is quite an attraction to see if you have never been there before.