Google Images 'HMS DREADNOUGHT'
You will find several contemporary B&W photos which are inconclusive as to the colors at the waterline. However you will also find some contemporary watercolors and colored post cards which appear to show a black waterline. Add to that Robert's Anatomy of the Ship also shows a black waterline. You are pretty much free to paint the waterline as you choose and dare anyone to tell you that you are wrong.
Black boot topping came into place around the turn of the last century and into the teens as oil replaced coal as fuel. Oil scum on the harbor's surface created a 'ring around the tub' appearance that coal dust and wood ash did not. It was more persistent and didn't just wash away. In order to keep a smart looking ship, captains went to painting a black ring at the design waterline +/- a couple of inches to hide the scum.
Also during the Victorian period there were several navies which painted colored "cheat lines" at the waterline which were largely decorative. I've seen paintings of bright red and white lines on black hulls. Pretty but not as effective in hiding scum