From the Magic Kingdom you can get to the resorts around that lake via water taxi. There is one water taxi that will take you to the Poly and/or the Grand Floridian (also the Momorail will take you to these places). Another water taxi will take you to the Contemporary/Wilderness Lodge and another to the Fort Wilderness Campground. There is even another water taxi that goes from hotel to hotel (ie.Cont/WL/FWC) without going to the Magic Kingdom.
Around the other lake you can get from the Hotels YC/BC, S/D, Boardwalk, to Epcot and MGM.
And from Epcot to MGM (not direct --with stops) via the water taxis.
These two lakes do not connect. For example: if you stay at the Wilderness Lodge or any of the Magic Kingdom resorts you can not get to any other park, but Magic Kingdom via boat.
Yes, if you are staying on property you can use any or all of these options. Even the Swan and Dolphin.
There is also a water taxi that runs between downtown disney and POR. It stopped running a couple mo. ago and I do not know if it is up and running again yet.
The Animal Kingdom, Typhoon Lagoon and Blizzard Beach you can only reach via bus/car.
FYI : You can not get from Hotel to Hotel directly via bus. I find it easiest to Get to Magic Kingdom or Downtown Disney and transfer.
If anyone had a better suggestion for this let me know.
I hope this is more helpful than confussing.