1971 merc 135 wont go in reverse

[Cord]

I have a 1971 merc 135 that I just had a new shift box and new cables installed on. However I have the same exact problem. It will go into reverse when its on land and attached to the water hose (trimmed down of course) but wont go in when its in the lake. What gives? I took it back to the mechanic and he sais he has to tear apart the lower unit and start investigating, but I ve already paid him alot of money for a misdiagnosis, Any ideas?

[boats.net]

If the shift cables and adjustment is good the it would have to be either the shift rod or inside the lower unit. Probably in the lower unit.