Hi Stephanie,

Since no one with more experience has responded to you yet, Ill give it a shot.
Disclaimer: I am not a pool, pond or landscape designer nor am I a contractor. I did not stay at a Holiday Inn Express last night. I havenÂt built a formal pond but plan to do so. I have built a couple of ponds at previous residences, one freeform using a flexible liner and one rigid liner pond to which I added a tiny waterfall run using flexible liner. I tend to over research and over analyze every problem or project prior to actually doing anything. In fact, most of my projects might never get past the research stage without a little "gentle prodding" from my lovely spouse.
My current plan is a partially submerged pond with a knee-high brick wall capped with coping cast from either the history stone molds or homebuilt molds. (I am gravitating toward the homebuilt method because I like the idea of being able to sit on the wall. The history stone curbing does not appear to be a comfortable seating surface.) I will place a statuary fountain on a submerged pedestal in the center. I intend to use concrete blocks for the submerged walls. I will dry lay the block and use a surface bond mortar like Quickwall®. The resulting wall is said to be stronger than one built using the traditional mortar joint method. I will pour a concrete bottom should I decide to tile the interior. (I like the idea but not the expense.) I may or may not pour the concrete bottom if I decide to go with a flexible liner. I plan to make the subterranean portion deep enough for fish and plants to over-winter at the bottom  at least 3 here IÂm told. One block course below ground isnÂt very deep. You might want to determine the depth necessary to avoid having to remove the plants before winter. That being said, I have a neighbor who actually enjoys moving her plants and fish into her basement each year. She keeps them in three large Rubbermaid containers equipped with small pumps and filled with pond water.
ThatÂs about all I can come up with. You should probably try a web search for some authoritative information from people who might actually know something. There are many sites available with lots of information on pond construction. (Often it's contradictory but thatÂs what makes free speech and the internet so great.) Here are a few that might help with your planning:
This page gives fairly detailed instructions for building a concrete block pond. It seems to be geared toward those who maintain a substantial investment in koi or other valuable piscine stock. I believe some of the recommendations are overkill for the "average" pond.
This one while not as formal as what you and I are considering is built with block walls. The construction is fairly well documented with photos.
Quickcrete® has instructions for a block walled pond. (lower half of page)
There is a GardenWeb forum dedicated to ponds. Perhaps some of the posters over there could be helpful.
Post some photos when you are done. (Photos of the process would be interesting too.)

Cheers,
ab

Thanks so much, ActuaryBoy! Your project idea sounds very similar to what I'm trying to do. I think making your own form would probably be better than the history stones one for seating - the edge is way to narrow -oouch! And if you are doing just a circle, I think 3' would be disproportionately deep because their circle is only 4' diameter. If you are mixing curves with straights for a different design though it would work fine. I wanted to do a moorish fountain in our last garden.
Anyway, thanks for both the links and the response. I was beginning to wonder if I said something wrong. All these talented and knowledgeable people and no one responded to my post. Back to the point, the links were very informative. I will have to employ DH to help and advise for certain. Thanks a bunch! Good luck with your project!