From the Polynesian you could either take the monorail to the MK or Contemporary and take a boat over to the WL or you could take the bus from the TTC.
I agree, we love the resort launch from the Polynesian! There is nothing so sweet as coming back "home" across the lake on the launch after a late night at the Magic Kingdom. (Of course, we are really water people, we were just out canoeing on a lake here in Southern Illinois yesterday.)
I would take the launch to the MK, and then catch the next boat to the WL. If you are in Tahiti longhouse though, I might consider catching the ferry or monorail from the TTC, since it is closer than the resort launch dock, and then taking the boat to the WL from the MK. Of course, the walk through the Poly grounds is lovely too, especially at night with all the torches burning. So, I guess it depends on what building you are in, and if you are looking for a fast way or an enjoyable, scenic (romantic?) route. Take the type of transport that appeals to you--boat, or boat and monorail. Have fun!