Why does regular bread yeast fail to thrive in pond water?
A few random thoughts.

I think that one problem here is that by using dried yeast you aren't really measuring growth or survival. Over a short time period much of the fermentation that you see probably has very little to do with viable cells. Dead cells in dried yeast act as permeable packages of enzymes and can promote fermentation.

Following on from that, it may be that components of the pond water can poison the fermentation carried out by the dead cells. 

Thirdly, if you are using frothing as a measure of CO₂ production it is possible that there are components of the pond water that collapse that foam by removing stabilisers (probably protein).

Next, Saccharomyces isn't an obvious choice for use as an aquatic micro-organism since it is adapted/selected for growth in high sugar concentrations. 

If I was trying to do this I think I would try to culture from the dried yeast under conditions where you do see activity, add lots more sugar (as if you were brewing), and look for actual production of yeast cells. Yeast settles out of suspension very easily so you might be able to pour off most of the liquid and add the cells to your pond. But I have no idea how well such cells will do in a pond.