there is no simple perfect fix that will equal a perfect varnish job.

Thin some varnish with thinner and use a fine pointed artist brush and fill the scratch. Several l coats if needed then don't look at it any longer.

Surface scuffs on autos can often be polished out but in my experience, it doesn't work well with varnish and since yours is deep, I would just fill it. I still remember being a new boat builder and can relate to the distress. Now I just accept the aches and pains of usage.