Here is a VERY long thread about soilfloc. Since your specific concern relates to a situation that few of us have encountered (bedrock in the bottom of the pond) you might want to instant message TJ by clicking on his profile in one of the posts on the page of the tread that I link to below, and see if he can talk you through whether soilfloc would have a chance of helping in your specific case.
You have it right, now is the time to do it right. Soilfloc is not expensive relatively speaking since hauling in clay, renting a sheepsfoot roller and paying someone to do the layers and compaction will be costly too. Doing the bentonite now and then finding out by fall that you will get to do something else again in the future because it didn't give the results you hoped for is expensive (and frustrating) too.
Soilfloc thread (there are several others too) 
