go visit http://www.aquascapeinc.com/findcontractors.... you can search by zip code for certified professionals in your area and then view their websites to get ideas. swimming ponds are very popular because they are lower maintenance than the previous pool and they are year round beautiful living creations with fish and plants. very cool stuff!
If you are looking for a more natural look, less maintenance, and some beautiful waterfalls I would definitely go to an ecosystem pond. Make sure you do your homework and find a great contractor. Aquascape's eco- system pond is the system we have found to be tried and true. We have even built several swimming ponds. The flash of koi and beauty of waterlilies and lotus can't be beat. The sounds of rushing water so calming...and there are no chemicals! (www.naturescapespro.com)
Personally I would go with adding a water feature to my in-ground pool. One of those sweet rock waterfalls or something.