His final act of heroism came on Labor Day, while trying to save his 12-year-old future step-daughter from drowning at the beach in Strathmere.
Costin's fiancé, Melissa Metcalf, told 6abc they planned a last-minute trip to the beach in Cape May County on Labor Day, Sept. 4.
They brought Metcalf's three children, Logan, Riley and Reagan Gallagher, as well as one of her son's girlfriends. They also brought Costin's daughter, Jocelyn.
But while Mark was swimming with Jocelyn and Reagan, tragedy struck. His soon-to-be stepdaughter, Reagan, got swept out to sea by a rip current.
That's when Mark went into action.
"He didn't hesitate to put her life before his own," Logan Gallagher of Newfield wrote on a GoFundMe page. "A nearby surfer grabbed my sister as Mark began floating off. Other surfers tried to reach him, but couldn't before a wave took him under.
"His life from start to finish was built on saving people from his tenure as a Medic in the National Guard to his final act of heroism on that beach," Logan said
Costin was one of four people who drowned on Labor Day Weekend at the Jersey Shore.
Metcalf described Mark, 42, as a spiritual health-guru, who loved to garden, according to 6abc. She called him a goofball who passionately cared for the people in his life.
Tributes poured in on Facebook:
Reagan's grandfather, Michael Cavaliero, wrote: "Mark used every bit of strength that he had left to push Reagan to a nearby surfer and then he went under and didn't make it out. He gave his life to save Reagan's."
And Mahogany Tyson wrote, "A hero he will be remembered as 🥺❤️🙏🏽. I couldn’t be more proud to call you brother… RIP"
To read or donate to the GoFundMe, click here.
Click here to follow Daily Voice West Chester and receive free news updates.