Wouldn't evaporation be the next nemesis if you did not pump water in? If you did pump the water back in, then you would have created a shallow well water fed pond. Most ground water ponds are filled due to a wide ground water source, not necessarily a single point like a spring fed pond. At least that is the way I would differentiate the two. So, If I had a wide ground water source pond that I wanted to stop the leak-down...I would seal the bottom, forego the trench and tile and place a shallow well nearby and pump to fill the pond. So long as you keep the pond level higher than the ground water level you would not experience ground water pressure over that of the pond water pressure, hence no overcoming upward pressures at the bottom seal.

If you have a single point fed pond, then Dave makes the point that there would be some pressure to it and sealing it off could be futile. At this point, a tile system as you suggested would need to be explored along with a way to keep the pond full.

Fish on!,
Noel