I just replaced my water pump on my 2016 Mercury Pro XS 150 hp and I’m having a devil of a time getting The shift shaft on lower unit lined back up with the shift shaft on the motor housing. I have the shifter in forward and the shift shaft on the lower unit and forward. And I can’t get the lower unit back on again Any suggestions would be MUCH appreciated. Thanks!

Are you sure it is the shift shaft and not the splines on the top of the drive shaft not lining up? Best advice is to have one person holding the lower unit and one person turn over the flywheel by hand until it slips in. If it really is the shift shaft, get a buddy to guide it in with a long pair of pliers while you hold up the lower unit.

I always turned the prop until the splines lined up and then lined up the shift shaft

If you dont have somebody to help a ratchet strap can be a big help.

It just took a second set of hands to wiggle the shifts shaft that connects to the powerhead. It was honoree but very glad I got it done and did not have to take it to a mechanic. It was the first time changing it on this particular boat. I bet next time it is much easier. Thanks for all the advice!

do it in neutral next time. i've never done one of those with it in gear.

Glad it worked. Assuming you are talking about the shift input and not drive/input, I do mine in neutral, and give a light jiggle/shake to the shift lever at the console in neutral (just a touch, not actually shifting) to get the slightest movement in the shift mechanism and it typically aligns and moves up.