The first mention I found for sea-fairing scouting was in a 1925 Girl Scout Leader magazine, listing the trainings available over the summer at the National Training Schools for Girl Scouts. Two classes were offered at the school in Marion, MA: Sea Scout Experiment and Sea Scout Camp for Girl Scouts (over 16). It's from courses like these, as well as the natural interest in sailing for the "older girls" in Girl Scouting, that the Mariner program grew. 

In 1932 newspaper articles were already telling tales

of Mariner Girl Scouts in many cities. It was described by

"national HQ" that... Mariner Scouting was a post-

graduate course of Girl Scouting. It opens a new field of 

adventure with the romance and the background of the sea

to sustain the interest of the older girls.

Officially launched in 1934, it sprang forth as a program

whose time had come. The Girl Scout Spring 1935 catalog

had:

• Mariner uniform: middy style shirt, skirt, slacks, shorts

• 2 hat styles (Gob and Beret) & black Mariner tie

• 2 manuals

• Unique membership pin

• Mariner Swimsuit

• Mariner Shoes

• Mariner sweater

• and 2 styles of Mariner flags

(one for the ship & one for parades)​​