Go to the aquascapes site. And order their video - they go all out to help you sell watergardens, because after all, it helps them, too.
TheMom has a good idea, too, in talking to existing customers to get them to to try a pond - I cut the first one a huge price break to give it a whirl, we had done a big patio there and I knew they wanted to do someting with water - I just called and said "How'd you like a water feature for cheap?"
We'll be doing another 3 this year, with the expectation of continued growth in coming years. We just offer it as another of our services.