How low can a helicopter legally fly over the water with boats below?
There's no minimum altitude over open water but the pilot must stay at least 500ft away from the boats, per 14 CFR 91.119:


  (c) Over other than congested areas. An altitude of 500 feet above
  the surface, except over open water or sparsely populated areas. In
  those cases, the aircraft may not be operated closer than 500 feet to
  any person, vessel, vehicle, or structure.


There's also a catch-all regulation that prohibits endangering other people, 91.13:


  (a) Aircraft operations for the purpose of air navigation. No person
  may operate an aircraft in a careless or reckless manner so as to
  endanger the life or property of another.


It's very hard to tell from the photo how far the helicopter is from the boats but if your friend believes it was closer than 500ft, she can report it to the FAA.