I got Frost Walker I when I enchanted some leather boots, and now I got Frost Walker II from fishing and added it to different leather boots that did not originally have Frost Walker. I used them on water and can I say I cannot tell a difference between I and II.
So what are the differences between the two enchantments?
If moving on ground (i.e. not falling, jumping, or flying), any still water blocks with air above within a radius of 2 + Level around the block being moved to will be turned into frosted ice.