If you want goldfish in the pond, it'll need to be 3ft deep, and if you want koi, it'll need to be 4ft deep or more. Unfiltered ponds can only be stocked very lightly and need to be quite planted too, any fish in the pond should also be fed very lightly- but in this day and age, filtration has so many bennefets for the pond and fish in it. I would say that if you have the money, i would definately advise getting filtration if you are planning on having fish in the pond.
Ponds 2ft deep or shallower are only really good as wildlife ponds, as such a shallow depth does not give the fish any really protection from predators and you also run the risk of losing all the fish during cold icey winters too (having more depth helps insulate the pond from the cold and helps aid the fish in their semi-dormant winter state).
When building the pond, you need to level the bottom and sides out with sand (if you soil is also dry or crumbly you may also need to add a concrete collar or brick sides to the inside of the bottom to prevent the sides collasping in over time) and add a layer of underlay to help insulate the pond under the liner.
In the olden days (like the victorian times) people used to line ponds with clay a lot, however this is a difficult process and can make the pond water look muddy- its also not very sturdy and the pond can widen and become shallower over time. Fast forward a bit in time, people started to use concrete to line the pond. This is still used today, however it is a difficult process and obviously makes it extremely difficult to alter the ponds position, shape or size in the future once it is done- it also needs to be very thoroughly washed. I don't know a great deal on concrete lined ponds though, i think the lime can be a problem. Now days people usually use pre-made plastic liners/tubs for ponds or use rubber liner with underlay.
