Can I change the watermark wording and opacity?

Yes. The ARC Watermarker gives you controls for exactly how loud the stamps are:

• Turn each element on or off — the cover banner, the "Prepared for [name]" personalization line, and the per-page (footer/header) stamp.
• Set the intensity with an opacity slider, from a subtle mark to a bold one.

That lets you strike your own balance between clear deterrence and comfortable reading. See Will the watermark affect readability?