Greetings from the US.
Well, in the US, if you were on an inspected vessel (one allowed to take more than 6 passengers for hire) it would be required.
I once helmed a warship (only for about an hour
)and the requirement was to keep her within either 1/2 or 1 degree of course...not easy to do. Obviously, if the compass deviation were off 2 degrees the result of a long ocean crossing
would not be good.
That said....are you on a sailboat? When you are out sailing, how on course are you? Do you account for set and drift? If you or your autopilot
are keeping it to within a degree, you are doing quite well and the conditions must be quite settled.
And true enough, the GPS is unaffected by the elements on a boat that cause compass deviation. Further, generally, you simply need to take a good hand bearing compass forward, away from metals in the cockpit
or wheelhouse and you can get a reading without deviation (generally).
BUT.. I am with YOU. What if....you got into a situation where you were slightly confused. The GPS gave one heading, your compass gave another and your autopilot
perhaps another. What if there were some interference
with the sat signal of the GPS and it was in error, how would you know? How would you reconsile the error?
What you are suggesting to do, build a deviation table, is a very prudent and...SIMPLE thing to do. It might be very illuminating...what if you found that you had a bad compass? Or that there was significant deviation on the boat? Or, what if you found your compass was near perfect, you woudl then have a standard on the boat that you could use for reference.
Get a very good hand bearing compass, and your GPS/plotter and tell your husband it's all about the girl! I am sure he is a smart guy and he will figure it out.