I would go with the piers, I am building a small house on the upside edge of my pond and am using them. I had a soils engineer out and he drilled a test hole that placed solid soil at 20' so I had a pile driver sink concrete pilings/piers and am putting the footing/foundation them now. Water runs through my soil almost as if the soil were not there, I would say like a river but, it's more like thousands of tiny streams. Dig any where on the property and within a foot or less I will hit water. In a couple of months that will move to 2'.
In your case the piers would also allow you to adjust for any soil shifting. It has always been my experience that clay expands when wet and again when dry, doesn't sound logical but, that's my experience.. The raised floor will cool you in both summer and winter. A slab is not always impervious to water either. The water will some times be drawn through the slab, hydro static action I think, and cause the surface to deteriorate.
If you decide on a slab look into a "mat foundation" or raft foundation.
www.usace.army.mil/inet/usace-docs/armytm/tm5-818-1/ch-10.pdf#search=�mat%20foundation� Probably more than you need there but it might help. I built one of these myself and it was not difficult just expensive because of the rebar cost.
