Moose wrote:From that picture it looks like individual footings, not one single mat.
What you are seeing are the concrete pedestals that extend from the foundation, those concrete pedestals are not the foundation. Think of them more as short concrete columns meant to transfer the load to the foundation. The foundation itself is below water and not visible. There is literally not enough space between the pedestals for each one to have its own spread foundation (or pile supported spread foundation). They are typically a minimum of 3'x3' Length by Width.
The other option would be they are each founded on single piles or caissons... However, there is a minimum spacing required between piles or caissons to avoid what is called "group effect", which is just a fancy way of saying that if the piles are spaced too closely together they will not act individually and their combined strength will be less than the sum of their individual capacities (usually approximately 3x the pile diameter)... From eyeballing the dimensions, it does not seem to be met in the previously posted photo( If they are caisson's then they would have to be almost as wide (in diameter) as the concrete cap at the top are almost touching in many cases.
Below is a typical spread footing sketch...
That widest section at the bottom is the footing, nothing else. The rest is considered a pedestal or pier.