The S.E.A. originated from the Adventurer's Club in Pleasure Island (opened in 1989)...that pre-dates Tokyo DisneySea (opened in 2001).
Actually, the Adventurers Club was an inspiration for the S.E.A. S.E.A. made it's first appearance at the 2001 opening of DisneySea. The
Fortress Explorations attraction gave guests the opportunity to join S.E.A.
Magellan’s restaurant served as the meeting place of S.E.A.
The
Tower of Terror was then themed to Harrison Hightower, a member of S.E.A.
It next jumped to Hong Kong for the opening of
Mystic Manor. The attraction contains many references to S.E.A in its queue. Hong Kong also has an
Explorer's Club restaurant that refers to S.E.A. (and contains props from the PI Adventurer's Club). Aulani has a reference to Harrison Hightower in a letter from Adventurer's Club Presiden Pamelia Perkins, but it does not mention S.E.A.
The Disney Cruise Line and Typhoon Lagoon all reference Mary Oceaneer as a member of S.E.A. At WDW, S.E.A. first appeared in
Big Thunder with Barnabis T Bullion as a member and in the
Skippers Canteen which features a S.E.A. dining room, S.E.A memorabilia and adds Dr. Albert Falls as a member. Jock Lindsay is also listed on the menu at his hanger bar as a member of S.E.A.
It is also important to note that S.E.A. predates the Adventurer's Club in time setting. The Adventurer's Club is always 1939 while S.E.A. is set in the late 1800's. So, it is possible that the Adventurer's Club is a later incarnation of S.E.A.