Michael

Your control box should allow some tightening of the lever throw. You'll need to take the cover off the control box by removing the screws, and, possibly(depending upon model of your control box) "popping" the cover gently off the dimple pins used as secondary fasteners.

This done, you will reveal the lever friction screws. Turn them clockwise to tighten. Do a little at a time. DO not try to shift without the engine running. Take the boat out for a spin, then see how you do.

G'Luck
Kevin