So would a a boat that finishes by performing this manoeuvre be acceptable:
Seeing as a boat can finish a race On the flags would it make any difference?
Or would the player be forced to traverse back around the buoys and move between them from the correct side??
The move you show in your graphics would be illegal. He would need to go back around and then pass the finishline.
This also holds true for any bouys during the race.
You can explain this (the hard way) by giving the passing of the bouys or finishline points. Passing it the correct way gies you 1 point and passing it the wrong way gives you -1 point. To make a legal passing you need to get to 1 point.
In your graphics the boat passes in the wrong direction first, so he is at -1 point. When he turns back to cross the finishline he gets 1point, but that only cansels out his minuspoint, so he needs to go cross the finishline once more in the correct direction to get to 1 point.