BWI/V shares buses with the S/D at all times. In the middle of the day, when it isn't nearly as busy, all the resorts share a bus....BW, S/D, YC/BC. Then, closer to park closing, they split up again...YC/BC has it's own bus.
The walk to MGM is very nice. The only time I don't like it is mid-day when it is hot/humid, or raining. Otherwise, it is quite pleasant. It takes about 15-20 mins...depending on which resort you are leaving from, and how quickly you walk. I have made it from the Intl Gateway to the MGM turnstiles in 18 mins but I was flying!!! In order to get to the walkway, just head to either the S/D or BW. It's actually six of one, half dozen of another which way you choose. The walkway runs along the canal, behind the BW, and leads right up to the main entrance area of MGM.
The boat is fun, but it can take awhile. It takes about 15 mins from the S/D to get to MGM. So, if you have to wait for a boat to arrive, you may have to add on an addtl 20+ mins to that time.
As far as the basic transportation at the BW, YC/BC...the buse do a pretty decent job. The only bus that I find I have to wait overlong for is the bus to DTD...and that has been the same at all the resorts. Those buses just don't seem to be as frequent as the regular park buses.