Is this a collection of screensavers that feature Waterstones branding and not necessarily an advert as such? (The special offers Kindles sold in the US feature different ads/offers for specific items and they change or are updated for new offers from time to time. They are shown as screensavers, plus they now have banner ads displaying on the home page).
If so, it's maybe a bit more like when you buy a mobile phone that has a themed, branded display rather than adverts? Usually phones have the option to remove this branding in the options somewhere, that should be an option for the Kindle too.... otherwise Waterstones should include the fact that's it's branded in the sales information.
If it's just a screensaver, you could you look in the
Developers Forum, you'll be able to see more information about jailbreaking, and could ask to find out if a screensaver hack would work, but you really shouldn't
have to do this. Like HarryT says, you do have the option to take it back to them and say you want a refund if there is no option to remove the Waterstones branding.