20 ft is not that far, some soils transmit water a lot better than others.is your land level or possibly sloped away from the pond?if it is sloped away its possible you could install the proper drainage system around your celler and get the water to come out before it penetrates the walls. i would want to definitely go with solid concrete walls as blocks are much more porus and more liable to let water thru.however there are products that can be put on them that make them almost as moisture proof as concrete. but even with concrete in some soils you will get a hydraulic effect that will force the water thru from under the floor or thru the walls. your best bet is to discuss it with a basement contractor that is familiar with your area and see what he would do.it is also possible to put in a curtain drain to get the water away from the area yu want to use before it gets close to your cellar.if you are on totally flat ground im not sure yu have a chance tho.if yu are just wanting a cellar for storm use yu could put a sloped floor with a hole for a sump pump in it. it will always be damp tho and yu will need to wire it and moniter the pump.