Was the reverse working before you change water impeller? If yes, you may have unscrewed the lower shift rod. You don't have to drop the LU unit yet. Instead, disconnect the lower shift rod from the upper shift rod. Now try to fully screw in the lower shift rod clockwise direction looking from the top. It screws in to the shift coupler located inside the LU gear box.
Next using the top of the LU unit where the lower shift rod protrudes as your reference, push down on the lower shift rod and the gear should engage (Forward). Check this by turning on the prop and should not turn. Mark the shift rod position with reference to the top of the hole with a red line or black marker.
Next pull up the lower shift rod all the way, again the gear will be engaged (Reverse). Check the prop by turning it and it should not turn. Mark the shift rod like before (a different color marker is preferred).
You now have the two max points of the lower shift rod, which are the forward and reverse. To find the neutral position, it should be half way between these two marks. Mark it with a different color marker. Test the neutral position by pushing the shift rod down half way between the two initial marks. Check the prop and it should be free wheeling.
Now you have all three marks, reconnect the lower shift rod to the upper shift rod. Shift the control handle to reverse and check if the lower shift rods is aligned with the neutral marking and prop is engaged and not moving. If not then adjust as necessary. There's an adjustment just below the carbs. Try following the shift cable mechanism and you should find it just below the carbs. Do the same with the forward position.
