For both WL & Poly, take the bus to the MK. To WL, you have the choice of a bus or a boat from the MK. To Poly, you have the choice of the Resort Monorail or a boat.
For BC, take the bus to DHS. From there, you have the choice of either walking (about a mile) or boat.
Anytime you're traveling between resorts you should allow 1 1/4 hours travel time. It probably won't take that long, but you may just miss the bus or boat, or there may be a delay to load wheelchairs, etc.
You can only transfer at an open park. If you're traveling in the evening and the nearest park has closed, you'll need to go to another park that's still open or to DTD to transfer. Bear in mind there's also a boat between OKW & DTD that you can use as an alternative to the bus, between 10 a.m. and 11 p.m.