Can a girl cox a boys' crew, or a boy cox a girls' crew?

Not in schools rowing. Under the SASRU rules, a coxswain counts as a member of the crew, so:

• A boys' crew may not be coxed by a girl.
• A girls' crew may not be coxed by a boy.
• At the SA Schools Championships, no boy may enter or take part in a girls' event, and no girl in a boys' event, including coxswains.

The RowSA Rules of Racing also treat coxes as crew members who must be bona fide members of the club. Junior boys' crews have male coxes and junior girls' crews have female coxes. Only mixed junior crews, and senior and masters crews, may be coxed by either gender.

You must also be a member of the same school or club as the crew. School club boats can't race composite crews.