Thanks for checking and verifying. At least someone else can see it!
I re-verified the cache and started a completely new save ... but no luck. Same behavior. I also noticed that the boat doesn't want to move if I use wood as fuel rather than coal; not sure of those issues are connected or separate.
You mentioned that the freely drivable boat uses car movement, so I spawned a car. My thought was that maybe, if the boat was broken, the car would also be broken since they used the same logic. But no dice. Car works.
If I uninstall Spaced Out, the boat also works as intended. So, it's something about that mod and the interaction between that one and this one ... but (for now) only on my machine. Sounds like this one might not be easily solvable.
All the same, I appreciate the time and support.