Definitely step should be slightly behind the CG. When planing and about to lift off, the plane is supported on the small area just forward of the step and should have slight positive angle of attack.

If step is too far forward, the plane will sit tail-low, at a too-high angle of attack, and may have too much drag to get up to speed and to a relatively level liftoff attitude. Have seen many "taxi queens" cured by moving the step aft. Of course, if you have enough power you can take off regardless, but high attack angle gives a fierce takeoff.

This assumes you have a single-hull seaplane or conventional plane supported on 2 floats. If you also have a tail float like some oldtimers, then you have essentially a hydroplane (think "Miss Budweiser") and will want the mainfloat steps a bit forward so the plane will be supported at 3 points.

-Dave