I visited a club at Greve in Copenhagen, Denmark.
They have a pond for waterplanes and boats, 115 metres long by 32 metres wide (380 x 105 ft) so it is unidirectional but the 115 metres is long enough and unobstructed enough as a runway.
Check it out on Google Earth at 55deg 35'49.10 N; 12deg 16'43.28 E
The main problem there is that after a few years it is unusable due to weed. A shallow pond will get clogged with weed quite quickly.
Do you have a solution to the weed problem?