I was going to put a geothermal system at our cabin. We have a large lake. Thought of putting the pipe coils in lake. Then changed to a pit idea. Dig large hole ( 10 feet deep) in bottom of the lake, then put intake pipe in hole so you would bring in lake water to system, cover with gravel to filter water. This would be just like a water well. The cold or hot water would then be put back in the lake at another spot to not affect the temp of incoming water. Then would also put an extra air hose in hole that would have lots of holes in it, so you could pump a lot of air through ( 100 cfm or more ) to clean the gravel of sediment when it would need cleaning. Was sure this would of worked but lake filled before we got to putting in. We have a lot of clay in our ground, do not need to worry about pond liner.
This was just my idea not a proven system so not to buy a lot of pipe or a radiator system that could be caught with fishing hooks. But could be changed over if it would have not worked. Also remember everything would need to be below the frost line.