From TARDIS Index File, the free Doctor Who reference.

For the Time Agent, see John Hart. For other individuals named Hart, see Hart.

Captain John Hart of the Royal Navy assisted the Doctor against the Sea Devils. The commander of the HMS Seaspite, now converted into a SONAR testing station, the Doctor had gone to him for help. (DW: The Sea Devils)

Behind the scenes

Decades after the broadcast of The Sea Devils another character named John Hart appeared several times in Torchwood.