Microsoft Word has a feature to insert a watermark, but not PowerPoint. Here’s how you can insert a watermark with the help of Custom Animation (Emphasis > Transparency):
First, you will need to enter the Slide Master. Click View > Master > Slide Master.
If you have a watermark image, insert it on the content master. If not, you can insert text or WordArt (the 2nd style) onto the content master.
Next, click Slide Show > Custom Animation.
Select the watermark, click Add Effect > Emphasis > Transparency. The default will give it a 50% transparency. You can adjust the setting according to your preference.
Set the Start from “On click” to “After Previous”. This will allow the transparency to start automatically.
Now, right click on the watermark and select Order > Send to back.
Finally, close Master View.