Selected Answer
There is no watermark in Excel, and if there was one its purpose would be to guarantee authenticity, not to safeguard data. Therefore my question. What is your purpose?
Guaranteeing authenticity would mean that you want to be sure that the data you are looking at have not been altered. If there was such a thing as a watermark in Excel, you might demand that the watermark should disappear when alterations have been made. If such is your requirement a solution might be possible, although, as Don has explained, Excel isn't designed to withstand persistent attempts at forgery.
If your purpose is to safeguard data you might demand that they disappear when the file's name is changed. Subject to the same limitations already mentioned, that too might be arranged. However, the idea hardly makes sense.
More to the point might be some precaution which screams a warning when data or formulas have been modified. One might think of a solution for that, too, all subject to the same limitations. However, if the task is not to allow unauthorised alterations then by far the better way would be to save the worksheet as PDF and sign the PDF. You can do that with Adobe Reader, it's free, and you can be 100% sure that no alterations have been made while the signature is intact. The signature is electronic and will be deleted automatically - with a very high degree of security, I believe - if any change is made to the document. In addition, a PDF file can be password protected too, making Fort Knox a push-over by comparison.
Adobe are the go-to people when you are looking for the security Excel doesn't offer.