Hadn't heard that, but have seen Jeremy Wade fishing for them on his TV series. I knew folks that lived on James Island, outside of Charleston whose house was on a salt water canal, and used to get dolphins swimming off their dock. I always wondered if dolphins came that far in, that sharks might too.
Land locked Lake Nicaragua has fresh water sharks. Apparently they were trapped there long ago when a large bay was cut off from the sea and gradually evolved to be able to survive in fresh water.