HI j'
Your watermark is likely already in Section Master.
If the watermark appears on every page, it's a master object. You can confirm this pretty easily:
With the document open, look in the Arrange menu, choose the Section Masters item, then Make Master Objects Selectable.
Then click on the watermark to Select it. You should see small handles (square or x) at each corner of the object.
You don't need to move that object; it's where it should be.
You do need to make the Table cells transparent by removing the colour fill in them or by setting the opacity of that fill to a value close to 0.

The example here is a lowly crafted one—a shape for the Shapes button with text placed in it—used just to illustrate the process.
Cells in Pages tables are colour filled by default (even the ones filled with white). Set it to No Fill (shown) or (if available) use a colour fill set to low opacity.
Regards,
Barry