I watched Enchantment on January 2nd on Polynesian's Beach. The main beach area was packed, but the area closest to TTC to the right of the Cabanas was absolutely deserted. We were the only people there. It's right here.
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Alternatively, I know you already mentioned it, but California Grill has an exterior balcony overlooking Magic Kingdom that could work for a more peaceful/quiet proposal, before, during, or after the fireworks.
The fireworks cruise idea also works great.
The Yacht and Beach club waterfront is also gorgeous and not crowded. It's right next to the uber fancy Monsieur Paul, so that could be a pre-meal spot.
Another uber fancy restaurant is Victoria & Albert's which could also be good for a pre-proposal meal.