Cardude,
The Kanzaki KM4A2 has a mechanical cone clutch. If you are able to get the transmission into neutral using the shift lever, the output shaft should spin freely and smoothly with no discernible end play.
Cone clutches can be difficult to disengage if the engine idle rpm is too high, or if there is any significant resistance in the drive line that keeps the cone engaged. The higher the resistance, the tighter the lock up.
I have included a diagram of the shift lever and its alignment/adjustment. Note that in neutral, the lever is exactly horizontal (in line with the crankshaft). Sometimes there is a reference mark on the transmission housing. Also, be careful with the washers under the end bolt. They are actually calibrated spacers that adjust the depth of the shift fork in the cone clutch.
If, as you state, the propeller shaft is hard to turn, it could definitely make the transmission hard or impossible to shift into neutral with the engine running. It could also stall the engine when shifting into gear at idle.
With the transmission in neutral, you should be able to rotate the propeller shaft by hand. Sometimes two hands, but not much more than that. If you need a wrench or lever to turn it, there is something wrong that will require correction. As you mentioned, maybe the diver missed something. With the transmission in neutral he should have been able to easily rotate the prop in both directions. Definitely check the stuffing box adjustment.
If you do need help with the transmission, you can contact the Yanmar Distributor for the Gulf Coast:
Laborde Products, Inc.
74257 Hwy 25
Covington, LA 70435
Toll Free: 1-800-628-9882
Manufacturers Offered: Diesel America, FPT - Industrial, Hatz, Mitsubishi - Industrial, Mitsubishi - Marine, Mitsubishi - Small Bore, SteyrMotors, Yanmar - Industrial, Yanmar - Marine
They advertise certified technicians on call 24 hours a day, 7 days a week.
Hope some of this info helps. Best of luck