Hi there! I just got back last night from my first stay at the Poly. I would recommend watching Wishes from the more secluded beach in front of the Tahiti longhouse.
Get off the Monorail at the Ticket and Transportation Center instead of at the actual Polynesian resort. Follow the signs to the "Walkway to the Polynesian. The Tahiti Longhouse is the building right along the water that is closest to the Ferry launch and the TTC. Directly in front of the Tahiti Longhouse is a very private beach with lounge chairs, a hammock, and a bench swing. There isn't a sidewalk to the beach but you can easily get to it. We watched Wishes from the hammock on the beach (while drinking a bottle of champagne) and it was perfect. Unless the wind is blowing too hard you should be able to hear the music (it won't be as loud as if you were at the MK) but in general you can hear it.
If you decide to stay for the Electrical Water Pageant, then move down to the main beach. You will have a better view there.