Make sure that the linkage is hooked up. Go into the engine room and see if moving the shifter from up above is truly moving the linkage below. Something may have become disconnected or broken. You can always move the linkage into forward right at the transmission and hold it there with vise grips if necessary, just shut down your engine as you get close to dock.
Make sure that you have enough fluid. Check the fluid level.
Make sure that the fluid is clean. While you are checking the fluid, wipe a little on your fingers. is it red or brown? smell the fluid, does it smell burned? if it is brown or burned change the fluid.
If you have really burned the fluid, not that uncommon, try this trick before you hand over a thousand + dollars to the repair man. You have nothing to loose and everything to gain. Drain your old fluid out. Add a 50/50 mix of new fluid/kerosene or diesel. Run in gear for 20 minutes, shift occasionally. Drain and fill with new fluid. Run in gear for 20 minutes, shifting occasionally.
Drain and fill with new fluid. You are done. Unless you really did a number on your transmission, this will work for a lifetime but you must check your fluid and keep it topped up like you were supposed to! You do not get to many chances to try killing your transmission before you will have to pay. I used this on in Mexico to help a stranded boat.
