On freshwater beaches run whatever mode you want that can run stable. (This is also true of the dry sand at salt beaches).
The Park modes will tend to give you more depth than the Beach modes, in general, so I would only run the Beach modes at salt sand beaches where you need the extra stability. Even though the beach modes will work at freshwater beaches, there is no reason to take the slight depth penalty doing so.
Pick the mode suited to the majority of targets you are seeking, just as if you were land hunting. For example, if going for deep clad/silver try Park 1. If going for gold and other small jewelry, try Park 2. There is overlap between the modes so Park 1 should snag you jewelry if its there, just as Park 2 will snag you high conductive coins. Park 1 is probably the deepest mode overall. Switch off between the modes to see which is doing better for you.
Also, be aware that even though there is no salinity to deal with, there may very well be mineralization depending on where the body of water is located, so be prepared to deal with that. Freshwater hunting from a detector setup perspective, is pretty much the same as land hunting, just wetter. HTH
