Magician:
I think for waves magnitude measurements you should use separate sensor. With current "heavy weight" (btw, how many kg?) one, most of the freq. range already suppressed, as load by it's nature very good LPF filter, and there is no way to recover information of the signals above cut-of freq.
The float doesn't actually move (or isn't supposed to) , only its measured weight changes so any LPF effects due to inertia are swamped by other effects such as the viscous effect of the water sliding against the surface of the float as it rises and falls. In water it's frequency response is probably in the order of 10Hz.