I think the core of this thread, is based around the challenge that awareness of relativity effects, have caused a lot of less than completely informed people (which includes most of us) to get lost here and there. Even to the point where it can seem that we are being told that NO motion is real, and that it’s ALL relative.
Perhaps the better question would be to ask what EFFECTS of spin, are dependent on the spinning object being a part of a larger system, and what effects are entirely due to the object and it’s spin by itself.
The answer is the same: So far as we can tell, the effects of spin don’t depend on any other objects. But it’s possible that they depend on the entirety of all other objects in the Universe.
Spin is relative to itself, unless it’s a point, or a line rotating around it’s axis. Since in your boat analogy, it is not a single boat be a fleet of boats making up the structure, and they are moving at different rates in respect with each other.
I believe that it has rather conclusively been demonstrated that the propagation of gravitational waves does not exceed the speed of light. If the α-Centauri stars were to unexpectedly crash into each other and explode into a diffuse nebula, the effect of the change in the system’s gravity would be “felt” here 4 years later, at the same time as we would be observing the collision. Just this week, a neutron star collision was detected by LIGO and the whole global astronomic community was able to train their telescopes on it to observe the visible (and extra-visible) emissions. The dissidents are standing on quicksand.
Ok, it was not “just this week”, the observational frenzy happened in late August. The announcement was yesterday.
But it brings up an interesting on-topic question. Two neutron stars are going to ridiculous spin. But as they close in on each other, they will approach tidal locking. So, does their spin, as they move into lock-step, affect their motion? Will synchronous spin have a braking effect on their mutual spirals (extending the passage) or an accelerating effect? What happens with spins that mismatch by a great deal?
Short answer: What happens is that a few astrophysicists get their grant application for more supercomputer time approved. That’s a very messy and chaotic (and yes, I mean that in the technical sense) problem.