According to the book Introduction to Scientology Ethics the Sea Organization is:
A fraternal religious order consisting of Scientologists who have pledged themselves to eternal service to Scientology and achieving the goals and purpose of the religion. The Sea Organization derives its name from its beginnings in 1967 when L. Ron Hubbard, having retired from his position as Executive Director in 1966, set to sea with a handful of veteran Scientologists to continue his research into the upper levels of spiritual awareness and ability. Initially, Sea Org members lived and worked aboard a feet of ships. Today the majority of Sea Org members bear a responsibility unique within Scientology. They are the only Scientologists entrusted to minister the advanced levels of training and auditing and the only individuals who may hold the senior ecclesiastic positions in the Scientology hierarchy. No formalized structure exists for the Sea Org. Rather, each member is subject to the established lines of corporate and organizational seniority and authority of the church organization in which the member works. Despite its name, the Sea Organization is neither an “organization” nor “entity.” It is, instead, the manifestation of a commitment.