There is no Disney bus service direct from resort to resort. The fastest way to go fro CSR to Yacht and Beach Club would be to take a taxi, but that obviously costs some $. Using Disney transport, the best way to go IMO would be a bus from CSR to MGM, from which you can take a boat to Yacht and Beach Club (scenic and relaxing, but slow) or you can walk along the path past BW and SD. The reverse will work too, as long as MGM transport is still running (up to about an hour after park closing). The walkway and boat dock are outside the MGM gates, so you don't need to have a park ticket to make this transfer.
You can do the same thing from the other direction via Epcot, but will need park tickets as the buses drop you at the main entrance and you have to go through the actual park and exit at International Gateway (between Great Britain and France) to get to the boat or walkway.
Another optionand the only one that will work if park transportation is done for the nightis to use DTD as your switch point. Take the CSR bus to DTD, then a second bus to Yacht and Beach Club. Just reverse it going back to CSR.