Question: In what year did the submarine that sank the Tsushima Maru open to public tours?

Context paragraphs:
Blue Badge tourist guide: Blue Badge Tourist Guides are the official, professional tourist guides of the United Kingdom.  They wear a Blue Badge to indicate their professionalism.  They are recognised by local tourist bodies throughout the UK, and by Visit Britain as Britain’s official tourist guides.  There are over 2000 Blue Badge guides in England, Scotland, Wales, and Northern Ireland, who guide at Britain's tourist attractions and cities.  Some guides run guided walking tours on themes such as Jack the Ripper, Harry Potter and The Beatles.  They are responsible for the regular 2012 Summer Olympics walking tours and are the guides for the summertime public tours inside the Houses of Parliament.

White House Visitors Office: The White House Visitors Office is responsible for public tours of the White House, for maintaining a facility where the public can obtain information about the White House, and for other White House events such as the White House Easter Egg Roll, Holiday Open Houses, Spring and Fall Garden tours, State Arrival Ceremonies and other special events.

Cascade Caverns: Cascade Caverns is a historically, geologically, and biologically important limestone solutional cave 3 mi south of Boerne, Texas, United States, on 226 Cascade Caverns Road, in Kendall County.  It has been commercially operated as a show cave and open for public tours since 1932.  Informal tours were run as far back as 1875 when Dr. Benjamin Hester owned the cave property.  The cave was known by the native Lipan Apache people who lived in the area prior to 1800.

Cooper–Frost–Austin House: The Cooper–Frost–Austin House is a historic Colonial American house, built in 1681.  It is located at 21 Linnaean Street, Cambridge, Massachusetts.  It is the oldest extant home in Cambridge and is owned and operated as a non-profit museum by Historic New England.  The house is rarely open for public tours, but private tours can be arranged during the summer months.

Ehime Maru and USS Greeneville collision: The "Ehime Maru" and USS "Greeneville" collision was a ship collision between the United States Navy (USN) "Los Angeles"-class submarine USS "Greeneville" (SSN-772) and the Japanese-fishery high-school training ship "Ehime Maru" (えひめ丸) from Ehime Prefecture on 9 February 2001, about 9 nmi off the south coast of Oahu, Hawaii, United States.  In a demonstration for some VIP civilian visitors, "Greeneville" performed an emergency ballast-blow surfacing maneuver.  As the submarine shot to the surface, she struck "Ehime Maru".  Within 10 minutes of the collision, "Ehime Maru" sank.  Nine of the people on board were killed: four high-school students, two teachers, and three crewmembers.

USS Bowfin (SS-287): USS "Bowfin" (SS/AGSS-287), a "Balao"-class submarine, was a boat of the United States Navy named for the bowfin fish.  Since 1981, she has been open to public tours at the USS "Bowfin" Submarine Museum and Park in Pearl Harbor, Hawaii, next to the USS Arizona Memorial Visitor Center.

Brisbane Open House: Brisbane Open House is a Doors Open Days event held in Brisbane, Queensland, Australia that showcases many of the city's buildings to the public.  Among the buildings included are historical landmarks, galleries, office buildings, museums, places of worship and bridges.  Well known buildings not usually open to the public are open for free public tours.  The annual event is usually held on the first weekend in October.

Open House London: Open House London is an event which promotes appreciation of architecture by the general public.  It is a part of the organisation Open-City and is best known for its annual Open House Weekend which is a two-day event held on one weekend each September throughout London since 1992.  The event forms a London version of the European Heritage Days ("Journées Européennes du Patrimoine"), a Europe-wide event that started as "Journées Portes ouvertes des monuments historiques" (Historic Monument Open Door Days) in France in 1983.  During the Open House Weekend many buildings considered to be of architectural significance open their doors for free public tours.  Heritage Open Days is a similar event covering the rest of England, and takes place the previous weekend to Open House London.

Dey Mansion: The Dey Mansion, (pronounced dye) located in modern-day Wayne, Passaic County, New Jersey, United States, and originally known as "Bloomsburg Manor," played an integral role in the American Revolutionary War.  Built by Col. Theunis Dey in the 1770s, it served as Washington's Headquarters on several occasions.  Today, the Dey Mansion is currently open year-round with guided public tours Wednesday through Sunday.  For more information and schedule of tours and events please visit Dey Mansion on The County of Passaic website or Facebook page.

Tsushima Maru: Tsushima Maru (対馬丸) was a Japanese passenger/cargo ship that was sunk by the submarine USS "Bowfin" during World War II, while carrying hundreds of schoolchildren from Okinawa to Kagoshima.

Supporting paragraphs: Tsushima Maru USS Bowfin (SS-287)
Answer: 1981