The sound could be amplified slightly but, it shouldn't sound that rough. There is something wrong with the motor/shaft that is noisy and struggling to move.
The ESC could be struggling to supply both motors. I would check to see if the motors are connected to the ESC in series or parallel and maybe check that all electrical connections are "good". Try disconnecting the noisy motor, then run the quiet one on it's own. That will give you an idea of how it should sound and how loud it should be.
Having not built this model yourself, you will have no idea of how it should/ did sound when first built or of any faults/ problems that have occurred or changes made during it's life. I would be tempted to remove all of the parts from the model and run them "on the bench" . Making sure that the ESC is compatible with the motors and is capable of running two of them.
Refering to youtube (again) may answer a lot of your questions. You may need to purchase new motors/shafts and compatible ESC, to get over this.
Ian.
Latest tests:
1> Motors not driving shafts - just normal motor whine.
2> Swapped motors and couplings to other shafts. (this made shafts counter rotate obviously)
3> Swapped motors back leaving couplings on shafts.
4> Disconnected motors in turn. (i.e. one disengaged from coupling) - both shafts are noisy.
5> Put everything back as was.
2, 3 & 4 - Made no difference. 5 - Tightened mounts taking care to align motors/couplings up with the shafts as best as I could (there was a bit if wiggle room on the screw holes) - noise still there but hesitancy on one shaft less. Maybe suggests an alignment issue originally.
I think your last couple of paragraphs are about right. The guy I got this from was really into trains, the boat was his first and and only and got for his Son 18 years ago. It was used four times in a local pond (so he claimed) and then unused since. So everything RC is 18 years old and likely left attended for most of that time. As you say I have no history with this boat which doesn't make anything easy for me. Thanks for your help.
Its definitely time to start my own build but I need to sort the ballast out on this one first - will leave this to my next post.