Yes it's easy to apply and easy to remove, but it takes an intentional act to remove a watermark. It's like the old adage - locks help keep the honest people honest. Any form of protection can be defeated with enough time and effort.
You have to decide whether it's more important to serve your visitors, or spend your efforts trying to thwart something you have no control over.
GettyImages uses watermarks and they're in the business of selling their images.
I would rather spend my time giving people a reason to visit my site than worry about someone stealing parts of it. That's really the only way you can stay ahead of them.