I never understand this, If you have never built one before, why are you trying to do this professionally?
Would you try to build a house having never done it before?
How about a Car, would you just willy nilly start off building cars and selling them to people?
The correct answer is you don't offer this service, because you don't know how.
The options are:
Refer someone who does
Tell the customer you dont do ponds
Sub the work to someone else.
If you choose to sub the work, this is the best way to learn, you are still involved in the process as you are the chief contractor and contact with the client, but you are using someone who is experienced in building (and pricing) ponds/water features.
After doing several ponds (6-10) subbing it this way, you can then take a stab at doing it on your own, the next time one comes up.