I have mild ear pain in the left eardrum after being hit by a water jet a swimming pool. bruised? ruptured? cautions?

Possible: Possible to have a simple "bruise" or a rupture Membrane. An exam is needed to tell you for sure. If ruptured, caution to avoid getting bacteria in the ear as you would be more at risk of getting a more serious infection. In fact if ruptured, it is recommended to use sterile Ear drops (usually for the eye) in the ear canal. Best wishes.