We love water and boats, so we would take the resort launch from the Poly to MK, then walk about 50 yards to the right and get on the boat to the WL. If you prefer, you could monorail to the MK, then catch the boat to WL. The resort launch is so relaxing. I haven't been on the WL boat though.
The only time I would take a cab would be if the MK/fireworks were getting out just as you wanted to get on the boat from the MK to the WL. (probably won't happen for an early dinner)