Yup, you might get by chincing on the deep part but unless you are the National Spot Landing Champion, you can never have enough surface area. The consequences of overshooting or missing the bull's-eye are breakage. Plus, float planes are notorious for hooking on either take-off or landing and you will quickly find yourself pointing in a different direction than you started.

It would be easier to build a runway next to a lake rather than the other way around.