With the image in Inkscape, I will type whatever text I want to use.
You can see I tried a lot of different fonts till I found something I liked.
Usually there is not much to a watermark. If you want, you can leave it simply as white text.
Experiment with different options, such as lowering the opacity (Fill and Stroke (Ctrl+Shift+F). Move the opacity slider. You can also copy the text, change the color to black, blur it (move the blur slider) then put it behind the text for a drop shadow.
You could do something a bit more obvious like creating a rectangle that goes across the screen with your text inside. You can copy the box as well to change black for a drop shadow.
Also, if you plan on doing something that goes across the entire screen (the the rectangle above) make sure that it is long enough to cover your largest photos.