Depends on country
British includes colonies
http://www.theshipslist.com/Forms/BMD.htm
GRO Births at sea from 1837 to 1965 are available on 56 microfiche; however these are only indexes which contain names, dates and in some years, ship names. LDS Fiche numbers 6137268 - 6137322 The LDS Family Search Library Catalog will identify which fiche relates to which year.
marriage at sea by captains prohibited in us and uk
In 1961 the three mile limit was in place changed to 12 miles much later..