Erin Brockovich Boat Arrest -- Cop Says She Slapped Her Husband ... Threw Cell Phone Into Lake

Erin Brockovich has officially been charged with drunk driving a boat last week -- after police say she threw a cell phone into the water, slapped her husband, and openly admitted she was a TERRIBLE boat driver.

TMZ has obtained the police report from the incident last Friday on Lake Mead in Nevada -- and according to the doc, Brockovich and her husband were fighting loudly on board their boat when a game warden approached to investigate the situation.

According to the warden, he observed Brockovich throw a cell phone in the lake and then slap her husband ... which is interesting ... because Brockovich's husband filed for divorce last year. (The divorce has since been called off.)

The warden says he asked Brockovich to dock her boat but she stated that she was "not very good at driving" it -- so the warden helped guide it in himself.

At that point, the warden suspected Brockovich was under the influence -- claiming she smelled like booze and was slurring her speech. According to the warden, she also blew a .186 and .190 on the breathalyzer ... MORE than double the legal limit, which is .08.