Does anyone know where the Sea Scouts came into this?
No - but I do now. The Internet is a wonderful resource for information on almost any subject you can think of.

http://pressinfo.co.uk/seascouts/what_is_sea_scouting.htm
Sea Scouting has been a part of Scouting since the Movement started. The founder, Lord Baden-Powell, makes many references in "Scouting for Boys" to Sea Scouting, and to his adventures with his brother Warington, who, he relates was "both a sailor and a boy at heart". In his message to Scouts in "Sea Scouting and Seamanship for Boys", Warington Baden-Powell makes this important observation: "A Sea Scout must be a Scout; therefore I take it that you know the Scout Promise, Law and Regulations. Sea Scouting is simply a branch of Scouting"