Depending on what time of day your PS is, there are lots of options for getting around.
The rental car would be the quickest, if you don't mind driving.
You could also take the launch from the GF to the Magic Kingdom, and from there catch the bus directly to the Yacht Club.
Or take a bus from the GF to MGM, and then take the water taxi from MGM to the YC.
If the PS is for dinner and after park closing time, I'd probably go with the rental car