I am not sure what you mean by neutral throttle. If that means you use throttle only, that may not help defining the problem with the shift...since the shift would be disconnected.
You can do the following out of the water without the motor running. You will need someone to turn the prop for you. If someone does not turn the prop, you can bind linkages and break one of the plastic levers. They are no fun to replace...
Disconnect the throttle connection at the engine. While someone turns the prop backwards, try to shift into forward. when you do the prop should lock up. If it seems any easier to shift, then your throttle cable may not be adjusted correctly, causing tension in the shift box. If it does not shift any differently, then you could have a bent shift cable.
If the shifting is still difficult, disconnect the end of the shift cable where it connects to the lever on the motor. Now try to shift. If you can feel a significant drag, you probably have a bad cable. Make sure the cable ends are not getting caught on something at the motor end. Normally, the operation should be fairly smooth, except for the forward or reverse detent.
Mike