Is underground electrical cable underwater capable?
Absolutely not.  Underground rating presumes physical protection by the surrounding earth.  

Undersea cable is designed to be laid on the sea floor.  You may have other threats like water-moved rocks, or the footfalls of people or animals that even undersea cable is not made to deal with.   There might be no cable rated for that duty.   

If the cable is buried so it is both underwater and underground (in reasonable soil near the cable, i.e. not sharp rocks; but then rocks or something above to stop stream bed erosion) -- that would be fine.