ZF is a wonderful transmission builder
for BMW Mercedes Corvette and Sea Drive.
What’s weird is it’s forward which is not suppose to work if it’s your Sea Drive Clutch.
I think it could be a cable issue and easy to resolve. There should be two cables
coming into the engine room on a twin clamp. Assure the cables
are snug there. Disconnect the shifter cable at the ZF with help from the helm assure the cables run smoothly.
If everything is fine there it could be water
in your ZF oil
. They swell have no place to displace an Overfill. They get abused used as generators in neutral still spinning with no load.
Lastly it could be the prop or the anode.