One comment I would add:

I believe you have to be careful to not give the impression that you are going to be pinching every penny and/or bargaining the contractor down at every chance.

I'm not at all saying to give them an open check book, but you have to be clear about what you want and what they are going to deliver. Clarify what the unknowns are for your specific pond, especially in the contractors mind, so that you know where you will get into extra dollars.

Pinching pennies during the construction could be a long term mistake if you get a crappy job done.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."