I think it's not possible. A watermark is intended to remain whatever you do with the file. You could imagine a specific layer within InDesign and export your PDF with layers preserved (1.6+).
So the user can hide the layer to see. But as he can hide it, your pseudo watermark won't print neither.
Good luck and feel free to come back to us if you find something working.
For the attached script for force print the watermark, could that be attached in indesign?
Here, you can add a watermark not screen visible but printable and you can add a password to your pdf to avoid modifications (delete the watermark for example).
Create a layer, call it 'Watermark', and place your watermark on that layer.
Create a PDF preset, call it 'Watermark', check 'Non-printing objects'.
When you export the PDF, uncheck the 'Print layer' checkbox in the 'Watermark' layer's options. Simple to script:
app.documents.layers.item('Watermark').printable = false;
then export the document.
When you print the document, check the 'Print layer' checkbox in the 'Watermark' layer's options, then print.
I think our friend wants the opposite. he wants the watermark to print but not to display on screen
Here is the issue
Ah, of course, I should have read better. Thanks, Loic.