Check Excel help for "watermark", and you'll find this:

Mimic a watermark in Excel
Watermark functionality is not available in Excel. If you want to display a
graphic on every printed page (for example, to indicate that the information
is confidential), you can insert the graphic in a header or footer. This way,
the graphic appears behind the text, starting at the top or bottom of every
page. You can also resize or scale it to fill the page.

On the View menu, click Header and Footer.
In the Page Setup dialog box, click Custom Header or Custom Footer.
Click in the Left section, Center section, or Right section box.
In the row of buttons in the Header or Footer dialog box, click Insert
Picture and then find the graphic you want to insert.
Double-click the graphic to insert it in the header or footer section box.
To resize or scale the graphic, click Format Picture in the row of buttons
in the Header or Footer dialog box and then, in the Format Picture dialog
box, select the options you want on the Size tab.

Notes

Changes to the graphic or graphic format take place immediately and can't be
undone.

If you want to add blank space above or below a graphic, click before or
after &[Picture], and then press ENTER to start a new line.

To replace a graphic, select &[Picture], click Insert Picture , and then
click Replace.
Before printing, make sure the header or footer margin allows enough space
for the custom header or footer.

To delete a graphic, select &[Picture] and then press DELETE.