Will the watermark affect readability?

No. The stamps are meant to mark the copy, not ruin it. The banner sits on the cover, the personalization line is a single discreet line, and the per-page stamp is a small footer or header — none of it obscures your text. And because you set the intensity, you can keep it subtle.

Reviewers get a clean, comfortable reading experience — which matters, because a copy that's annoying to read is a review you won't get. This is the advantage of watermarking over DRM, which often stops reviewers from opening the file at all.