Forgot to say the coin dredge will only reach to 15 feet down but in your case that should be plenty. Also it creates it's suction by pumping water so it is capable of draining a pond quickly. If you don't have a reaspectable flow feeding the pond you may only be able to work a few hours a day and wait for it to refill so you don't drain the pond too much.

[Linked Image from img.whynotgif.com]
[Linked Image from i68.photobucket.com]