Welcome to the forum!
Even if you don't break thru into the sand, I would see about renting a sheepsfoot roller to compact the pond bottom after you've gotten it to the depth and contour that you want. If no sheepsfoot roller is around, I'd fill the scraper and max out the air pressure in the tires, and drive all over the pond bottom a few times to compact the soil.
Another thing, if you don't break thru the sand, I would dig some test holes to see exactly WHERE the sand was, and make sure that I had 24" of compacted clay over it. You could dig down to 11'-6" and only have 6" of clay over the sand....... That might stay sealed, and it might not.
The other option would be to continue deeper than the 12' where you think the sand is, then re-fill and re-pack clay over the sandy area. Deeper is better to hold more water in reserve. If you lose 6' of water due to a dry spell, your fish won't have that much water to swim around in.
