She is best known for playing headmistress Mrs Bassinger in the long-running BBC school drama, Grange Hill, from 2003–2006. The character's first name has never been revealed, but according to the sign outside her head teacher's office it starts with "S". Mrs Bassinger was not seen during the 2007 series, her function in the programme taken over by Cathy Tyson as deputy head, Miss Gayle. It was not known if Boatswain would return to Grange Hill, however the show has now been cancelled.
She has also appeared in Life on Mars and The Royal. In 2013, she joined the cast of popular Channel 4 comedy drama series, Shameless, for the final ever series playing Patreesha St. Rose. She also played Eileen the Crow in the 2015 video game Bloodborne.
Boatswain joined the cast of Channel 4 soap opera, Hollyoaks as regular cast member, Simone Loveday on 20 February 2015. Hollyoaks aired a Come Dine with Me special week to celebrate their twentieth anniversary, with Boatswain as one of the cast members hosting.
