The Mast Pond and
Boathouse 6 was designed by an officer of the Royal Engineers, Captain James Beatson in the 1840s, and was one of the first examples of a brick building erected around an internal metal frame.
Its purpose was to build and repair the small boats carried by large warships, a role it continued to fulfil right through until the 1970s despite extensive bomb damage during WWII.
These days it houses Action Stations a "high-tech, interactive indoor attraction" aimed at the kids. Most of the contents are beyond the scope of this site (and indeed its webmaster).