Granddamper
A granddamper is a grandparent who is damp or throws water on other people. This is a gender-neutral term so it can be used for grandparents of any gender. If they are throwing water on other people without their permission, they are a douchebag. If they are a grandparent who is damp, you should probably be concerned, unless they deserve it.
My grandma is such a granddamper in the summer. I'll never forgive her for drowning me in fifteen buckets of water!