Possible Clues:

• Old war subs
• WWII craft
• They scared Churchill, by his own admission
• They hid beneath the surface
• World War menaces
• 1940s blockade enforcers
• Battle of the Atlantic craft
• University School after characters in 33 down - in the sink business, they rarely put their heads above water
• Subs supply to a sub
• German submarines of WW11