I dug a pond 100 feet in diameter, average 5 feet deep with my JD350 dozer in one long summer day. That's about like a D3 I think and it's only less than 1/4 acre. A larger pond will take disproportionately longer since you'll have to push the dirt farther. I think it would take me about 20 to 30 days to dig a one acre pond with that thing. I dug a 1/2 acre pond with a D4 dozer in two long summer days. The fastest digging I did was with a 36,000 pound Komatsu tracked excavator that I rented. It had a 2 yard bucket and about 36 foot reach I think. Even if I had to move the dirt two or three times it was faster than even the D4 dozer. I can't say how long it would take to do a one acre pond because I used it to make two ponds larger so I was digging in water. That is slow because you almost never get a full scoop. As far as I'm concerned that will be my main tool for my next pond. Dig when the dirt is dry so you can stack it high and move it more than once when I have to. I like doing it myself for the fun of it. I think any backhoe whill be too small and take too long. After using backhoes and excavators ranging from a 1/4 yard bucket to the 2 yard bucket I'd never attempt a pond with anything smaller than 2 yards per scoop with the longest reach you can get. Bigger would even be better. I managed to do my own ponds because I was just digging a hole into the water table and letting it fill up. Like other advice given here, if you are doing anything complicated like trying to build a pond above the water table or building a dam you better get professional advice at least if not help. My 2 cents.

Gotta get back to fishin!