Kelly Gordon remembers her first time on a yacht in 2007: She climbed up the metal ladder rungs, stepped one foot on the back deck and caught a whiff of saltwater air.

"I didn't even know port from starboard," Gordon, 42, tells CNBC Make It. But for the next four years, she worked toward becoming a yacht captain, largely on summer breaks from her full-time job as an adjunct chemistry professor at Carteret Community College in North Carolina.

Today, she's a superyacht captain for "ultra-high net worth" boat owners. She typically works with one client at a time, and is currently based in Palm Beach, Florida, stationed aboard a 108-foot yacht, she says.

In 2019, Gordon started cataloguing her adventures on Instagram. Her videos resonated with viewers outside the yachting world, and content creation is now her side hustle, she says: She spends, on average, five hours per week on it, whenever she's off-duty.

DON'T MISS: The ultimate guide to earning passive income online

Her side hustle brought in $124,000 in revenue last year, according to documents reviewed by CNBC Make It. That revenue includes money from sponsorships, public speaking gigs, ads and affiliate marketing on her social media, and merchandise sales, she says.

Gordon uses much of the revenue to pay a video editor, PR manager, speaking coach and three other part-time employees. She isn't yet pocketing any of the money herself, she says.

Still, she has big goals for Captain Kelly J. Gordon, the business. Here's how Gordon built her side hustle, and how she plans to grow it.

From classrooms to boat decks

Gordon discovered yachting at age 26, during her first semester of teaching: She was invited by one of her students to a wedding reception on a boat, and didn't know anyone else there, she says.

The captain seemed overwhelmed by the number of guests, so she offered to help him set up. Later in the night, she worked up the courage to tease him, "I bet I could drive this thing."

The captain scoffed, but when Gordon showed up again the next day, he agreed to mentor her. He insisted that she spend time in every section of the boat, from the deck to the engine room, so she'd know how to relate to the crew if she ever became a captain herself, she says.

The mechanics of the yacht felt like operating tractors on her family farm in Indiana, says Gordon. And being on the water offered a peaceful respite from her mental health struggles: She dealt with depression and anxiety throughout her 20s, and contemplated suicide before the wedding reception, she says.

Instead, she made an appointment with a psychiatrist and started taking anti-depressants, she says. She became a full-fledged boat captain the following year, and balanced yachting with her teaching role until 2018 — learning how to maintain her mental health over time.

"[Age] 39 was when I was like, 'OK, I got this figured out. I know how to do this ... I know I'll be happy. I know how to do life,'" says Gordon.

An accidental social media following

In 2019, Gordon's two siblings convinced her to join Instagram and Facebook so they could stay in touch with her more easily.

One of her early posts, about a trip up the Mississippi River during the Covid-19 pandemic, drew comments from outside her friend group, she says — so she started sharing more, including videos about mental health.

Many entry-level roles on yachts are filled by young people, often taking a gap year between college and looking for a job. They're away from home, friends and cell service for long stretches of time, and it's hard to consistently get along with your co-workers when you live together 24/7, says Gordon.

She only realized her videos had traction beyond the "yachtie" community in 2021, she says, when a follower from France wrote that the content had inspired her to keep fighting through a severe illness.

"That's when I thought, 'I've got something here,'" Gordon says.

'I'm hoping this will be my retirement job'

Gordon currently spends eight to 16 hours per day on her client's yacht, five to seven days per week. She posts to social media once each day, filming when she's not actively working.

Her side hustle brought in nearly $24,000 last month, outpacing her highest-earning month of last year by roughly $4,000. Her Instagram has more than 10,000 followers, and she recently started a podcast about mental health, boat safety and yachting careers.

Gordon's goals: Earn enough to pay herself at least a small income by May 2025, and double her revenue every year until she reaches $1 million annually.

Her plan involves growing her current projects to earn more money through partnerships and sponsorships, and charging more for speaking engagements instead of occasionally doing them for free, she says.

Gordon also doesn't plan to leave yachting for the foreseeable future, she says: Her appeal to followers depends on being onboard, and she loves being on the water every day.

"I'm hoping this will be my retirement job ... traveling around the world, showing up at events, inspiring people and continuing with my social media," says Gordon, adding: "I'm not done running boats anytime soon."

Correction: This story has been updated to reflect that Gordon spent four years training to become a yacht captain, learned how to maintain her mental health throughout her 30s and currently works five to seven days per week full-time.

Want to make extra money outside of your day job? Sign up for CNBC's new online course How to Earn Passive Income Online to learn about common passive income streams, tips to get started and real-life success stories. CNBC Make It readers can use special discount code CNBC40 to get 40% off through August 15, 2024.