I have a BAYLINER Ciera 3055 with Bravo one outdrives that are having shifting problems. When I disconnect the cables from the shifter at the engine the shifter works easily but in normal configuration it is really hard to shift. The cables from the engine to the drives have been replaced. That helped some but not enough. The mechanics at the marina say I need to replace the shifter assembly but I’m not sure about that. If I disconnect the shifter cable to the drives at the engine should I be able to manually push and pull the cable and tell if there is a problem in the drive?
any help would be greatly appreciated.

Disconnect the shifter cable at the throttle/shifter lever. Then disconnect the shifter cable at the shifter plate on the engine. The cable should move freely by hand by pushing/pulling it back and forth.
Operate the throttle/shifter lever. That should move freely back and forth.

Being the upper shift cable moves freely and the lower cables have been replaced, I agree with the marina mechanics.
You need to replace the shift linkage assembly inside the outdrives. My Bravo 1's did the same thing years ago and after the shift linkage assemblies were replaced, they shift like butter.
Main part that wears out is #14 https://www.mercruiserparts.com/bam/...y/30966/952/80

What wears out on #14?

I just had mine out and after reading this I wish I knew to replace those parts. Mine were decent and I cleaned them up but replacing would have been better. I think for #14 the socket that the ball rests in starts to mushroom and maybe that's what makes it hard to operate? There's also a particular measurement you use to setup the cable from the drive to where it attaches at the bracket. There's a tool that you use. Could be yours is out of adjustment.

Just to be clear...the shifters/cables work fine and freely when not connected to the engine/outdrive? Then Objective Evidence indicates the marina guys are correct. There are YouTube videos describing this exact thing.

I thought my shifter cables were fine until I took my transom assemblies in for rebuild. Guy tugged on them and said to replace them. I disagreed, then he pulled a new one off the shelf. The slightest resistance in the cable multiplies and makes it harder to shift. They are like new now.

The detent ball fits into #14 and over time it enlarges the the area where they make contact

Well that was an informative video. So not only should you replace #14, but #16 and #4 as well. Although I personally never done it. It looks like, in the repair manual, you remove 14 with 16 still attached turning clockwise and it comes out as one assembly. So you don't have to remove the drive. Is that correct?

So nowhere in any of my manuals does it show this to be a preventive maintenance service. How long does the linkage and detent assem. last in your opinion? Or when shifting gets hard(er) this is one spot to check?
Thanks....

My boat is a 1997 and drives were original when I purchased in 2007. I had one side done in 2007 or 2008 because of hard shifting shortly after I bought it. I had the other one the next year because of the same. They have both been fine since. I paid to get it done and don't remember exactly what parts they replaced. I think it came as a kit with multiple parts though. I had the lower shift cables done at the same time each one was done, so my drives were off anyway.