"My cousin was in the Navy and I had a chat to him. He talked about travel and playing sport for your service. And I love being on the sea."
She joined initially as an RNZN steward, and changed trades to Military Police (MP) in 2016.
"I like the idea of enforcing the law, ensuring discipline, and always doing the right thing," she said.
"I also like being the problem-solver, the person people come to for help."
At Devonport Naval Base Marsh operates in a similar fashion to a civilian police officer, including investigative work and responding to incidents and emergencies.
She also deploys as a Military Policewoman for naval operations in the Pacific, or for tri-Service operations around the world.
"Some ships, like the frigates or HMNZS Canterbury, have permanent postings for NZDF MP personnel, while other ships request them for operations," she said.
"We carry out Customs duties, and border and regulatory work. We generally have other jobs while at sea, including boarding ships, damage control and whole ship activities."
NZDF Military Police work alongside New Zealand Police to gain experience, and often train with them.
"We do Disaster Victim Identification courses, Diplomatic Protection Officers courses, emergency vehicle operator's courses, and some of our MPs are defensive tactics instructors at NZ Police College," Marsh said.
"We work under different legislative rules but there are a lot of similarities. When you're out and about with police, we speak the same language."
It's not a job for the faint-hearted though.
"It's pretty challenging. I came from a support trade, and this is a lot more physical. It's mentally challenging as well. When people in the military ask me what Military Police do, I ask them what they think we do. People often perceive it differently."
She will deploy soon to the Middle East, in support of NZDF operations there.
Sport, a great incentive for her to join the RNZN, has always been there. She plays for the RNZN and NZDF softball and football teams, and has represented the NZDF overseas.