Any merit badge or award can be earned outside of summer camp.
It's up to the instructor to self-police his or her own qualifications, same as being a MBC. The BSA guidelines require that the instructor be an "experienced boardsailor". To my knowledge there is no certification for board sailing instructors. Lots of Scouts in the Portland area take lessons and sail in the Columbia Gorge and earn the Award, and other guys I know have earned the Award on family vacations to sunny beaches.
The Board Sailing Guidelines material is copied below (from the link at USSSP).
Board Sailing Guidelines
(Found in Section V of Camp Program and Property Management, No. 90-920)
Whenever Scouts or Explorers participate in boardsailing activities, the following guidelines and practices should be followed:
1.A responsible adult must supervise all board-sailing activities. The supervising adult must be experienced and qualified in water safety (BSA Lifeguard, Red Cross Advanced Lifesaving, or YMCA Senior Lifesaver) and must be an experienced board sailor or use assistants with such qualifications.
2.Only persons who have successfully completed the official BSA swimmers test in the current year may participate in a board-sailing activity.
3.All participants must present evidence of fitness assured by a complete health history from a physician, parent, or legal guardian. The adult supervisor should adjust all supervision, discipline, and protection to anticipate any potential risks associated with individual health conditions. In the event of any significant health condition, an examination by a physician should be required by the adult supervisor.
4.All participants should receive instruction in boardsailing skills and safety from an experienced board sailor.
5.All persons must wear an approved U.S. Coast Guard personal flotation device (type II or III recommended) at all times while participating in a board-sailing activity.
6.No one sails alone; always have a buddy on the water (in a boat or on another board). Board sailors must stay within view and easy rescue range of lookout in an appropriately equipped rescue boat.
7.No board-sailing at night, at dusk, or in rough water. Weather and water conditions must be known and understood in advance of any board-sailing activity, and weather forecasts should be studied. Exposure suits are recommended for cool water or cool weather.
8.Avoid swimming areas, fishermen, and underwater diving activity. Use designated board-sailing beaches when available. Stay out of traffic or channels that have heavy traffic.
9.All equipment should be safety checked and repaired as needed before each use.
10.When in difficulty, a person should stay with the board and not attempt to swim ashore.
11.All participants should know, understand, and follow the rules and procedures for safe board-sailing. The supervisor should encourage the individual exercise of good judgment and self-discipline, and assure that safety rules are fairly and constantly enforced.