Hi Novice, that line is a typical WL-marker, that you very often find on drawings - even technical ones to show the load waterline at standard displacement.
I was thinking of leaving the WL-marker off completely, but decided that, for pedagogic reasons to show it this way. I think it's the best way, with ships w/o a proper boot-topping.
it is not typical (the typical one is a line at both ends with a few short lines underneath) and the black line over the waterline position on your ship does the job just fine. IIRC these small 'markers' are not even allowed in sb, and I agree with that, as there is no reason to include them.