I believe the step to be way too far behind the CG. The step should be a maximum of 0.5" behind the CG. If you consider this for a moment, the plane is always trying to rotate around the CG, but is having a hard time doing so due to the position of the step being so far back as to act as a lever arm that, due to the force of the water at the far end of the lever, is keeping the plane from rotating. Therefore, when you are finally able to yank the plane off the water by exerting a lot of rotational force (pitch) by having a lot of up elevator, the pitch when the plane is released from the water is such that it does the backflip you describe due to the amount of elevator employed.

CF - What is your method of getting the drawing attached? I've tried to do this previously but the website won't allow uploading a Word document (.doc). What is the extension (.XXX) of your drawing?