The prison guard is patrolling the inmates.
He works as a prison guard.
He passed the exam and became a prison guard.
He served as a prison guard for ten years.
The prisoners were being watched by the guards.
監視員 refers to any kind of watchman or supervisor, whereas 看守 specifically means a guard who watches over prisoners.
看守 and 看護師 share the kanji 看 and sound similar, but 看守 guards prisoners while 看護師 cares for patients as a nurse.
Compound of 看 (look after, watch) and 守 (guard, protect).