I'm just a beginner as well so take my thoughts with some grain of salt...

Murray Abraham talks about in his dvd "aquaponics made easy" about people that want to use their existing swimming pool as a FT and why it is not advisable to do that. The minimum FT/GB-ratio is around 1:1. For 1000L you need 1000L of grow beds.
A olympic size swimming pool, 50x25x2 meters, is 2,500,000L. So a straight up aquaponics system won't really work. Besides, a fish to water ratio of 10l to 1 fish is usually standard. That's 250 000 Koi Karp in a olympic sized swimming pool... :P

But in nature the fish waste lays on the bottom of the lake and creates soil, very nutrient rich soil.
A more practical solution might be to have a fish farm in the lake and 2-3 times a year pump/dig/remove/drain the sediment at the bottom of the lake under the fish farm and use it to fertilize the land.

If I owned a lake I would introduce and raise high value fish like rainbow trout in the lake and plant a lot of fruit and nut trees around the lake and just sit back on the porch and enjoy my life. =)
Off course nothing would then stop me building a green house with a aquaponics system in it beside the lake...

By the way, if you're planning on buying some land with a lake on it, google the name Sepp Holzer for some inspiration.