Always, my MAIN concern is what happens if he produces a leaker? How much water loss has to commonly occur before he will come back and try to repair leakage?. How does he define a leaking pond? Get his promise in his contract.
For reference and for an idea of a pond as good as it gets, a well sealed, well compacted, core trench, dug out pond built in good hard clay in NW Ohio typically looses about 12" water maximum during dry spells between our climatic rains. Ponds in our area that typically go down more than that are leaking water due to seepage. Leaking ponds in our area will typically loose 2-3 ft of water during dry spells. Water table ponds (no core trench) in our area will go down up to 5-6 ft, some even more.
If at all possible, you do not want a leaky pond.
That is my main concern. Most everything else can be worked around and negotiated.
Other than that read Lusk's book Perfect Pond.. for suggestions. Others here may provide additonal advice.
