Our family sat and figured it out, and it was cheaper for us to get the Water Park and more option. We plan on hitting both water parks during our vacation, so we'll get good use of it. We "may" hit Disney Quest one afternoon, if we feel like it, but that's a big if.
Only you can answer if the money's worth it. Will you be riding the rides, or just sitting in a lounge chair all day? Will your children be splashing in the pools and sliding down the slides, or stick their big toes in and then just play in the sand?
Before kids, DH and I went on vacation with another couple to DW, and went to TL on our very last day. We were so mad at ourselves that we had left it so late--if we'd known what fun it was, we would have gone more than the one day! Likewise, our kids are little fish and absolutely love the water parks. So for us, it would be worth paying the full price admission. Buying the water park option was just a better deal.