The Sea Devils appear in two previous stories: The Sea Devils: Episode One (1972) and Warriors of the Deep: Part One (1984). They are cousins of the Silurians, who appear in several other stories.
This is only the second Doctor Who Easter special, after Planet of the Dead (2009), 13 years earlier. Unlike Planet of the Dead, no mention is made here of the holiday.
At just 47 minutes long, this is the shortest Doctor Who special episode in the series' history. It runs shorter than some regular episodes.
When Yaz and the Doctor swing from the Sea Devil's ship to Madame Ching's, The Doctor yells out, "Geronimo!" This was a favourite catchphrase of the Eleventh Doctor.
Zheng Yi Sao (1775-1844) born Shi Yang, a.k.a. Shi Xianggu, also known as Ching Shih, was a Chinese pirate leader who was active in the South China Sea from 1801 to 1810. "Wikipedia"