As I recall, a hex socket screw is accessible from the top side, under a little cosmetic clip about the size of a dime. Remove that hex bolt and drop the trim tab. Then look from underneath the AV plate up in the hole and you will see another bolt. That's your smoking gun.
If you have that bolt off, and it won't drop, your drive shaft is probably stuck in the bottom of the crankshaft. Numerous tricks on the www boating forums as to how to dislodge that.
One other thing is your shift shaft up at the front of the LU. Ensure that you watch that joint as you separate and ensure that the mid section shift rod spline comes out of the coupling...squirting with PB Blaster or other quality penetrating oil is a great idea. If you see the coupling, seals and all coming up out of the LU, stop and resolve that problem because if you pull the shift shaft stub up out of the LU, it separates from the "Cam Follower", pushed out of place by the spring behind the clutch dog spring and you usually have to open the gear box to get it back in position to restab the shaft splined stub.