When I bought this place I was told that the pond was 20' deep. I took posession (3 months after closing) at the end of February, and a week later the ice went off of the pond. I found out then that there was a Winterkill and all the large fish in the pond died. It turns out that the pond was only 7'-8' deep at max depth, and that was only for 1/4th of the pond at the maximum. I bought an electric aerator for the next winter and haven't had a problem with Winterkill since.
If it was my pond, I'd go to 15' depth, and plan on some sediment filling it back in a couple feet, but that's because I have sandy soil. I don't know how much sediment will build up in your pond. Personally, I'd be comfortable with 50% of the pond depth at 12' after the first couple of years sediment build-up. I believe you'll see the most sediment build-up the first year or 2, before all the plants get established around the pond.
With a 3/4 ac pond, the sides might get pretty steep if you go much deeper than 15'.
