Mel Stewart didn’t know there was such a thing as mermaid synchronized swimming clubs until a member of a local one asked to rent her Delaware County pool for practice.

But — because of their tails — the mermaids needed a deeper pool.

“Which is a shame because that sounded really cool,” Stewart said.

Stewart, a school administrator and former science teacher, and her husband, Gary, offer their pool and hot tub for rent on Swimply, a rental platform launched a few years ago in central New Jersey. Hopeful swimmers can see listings of accommodations and prices, just as for vacation home rentals. The concept is gaining traction.

During shutdowns early in the pandemic, swimming pools were popular additions as people stuck at home tried to create vacation-like settings. But most homeowners don’t use their pools every day, and maintenance can be expensive. High demand and supply shortages also mean homeowners who want a pool are stuck on waiting lists that stretch a year or more.

The Stewarts heard about New York-based Swimply on an episode of Shark Tank last year. Coincidentally, they were offering up their Newtown Square backyard to lots of friends, family members, and coworkers for private use during last year’s shutdowns.

They listed their pool for rent in April, but bookings took off after Memorial Day. More than 15 customers have reserved the space starting at $68 or $80 per hour depending on the day of the week. One person booked the pool to chill alone. A group of girls celebrated their seventh-grade graduation. Parents have watched children splash around for a few hours.

» READ MORE: Summer of 2020 was third-hottest on record in Philly as warm-night trend continues

Most swimmers have come from Philadelphia. For those with the means, renting private pools for a couple of hours can be an alternative to the city’s outdoor pools. Twenty-two of the city’s 69 more accessible, community pools won’t open this summer.

“We do have a very nice pool,” Stewart, 48, said. “We’re very lucky in life, and it’s kind of one of those things where we wanted to share our good fortune with others.”

According to Swimply, more than nine in 10 Americans don’t have access to a pool.

» READ MORE: Here are the city pools that will be opening in Philly this summer

Asher Weinberger, who founded Swimply with New Jersey native Bunim Laskin, said that when he talks about the concept to people he meets, “there’s a very visceral response.”

“It’s either super weird to them or super awesome to them,” he said.

He said he thinks more people will be looking for private alternatives to crowded public pools due to hygiene and safety concerns during the pandemic.

Day trips with children and small get-togethers with friends are the most popular uses. But pools on Swimply also host parties for birthdays, anniversaries, and baptisms; aqua therapy for seniors; swimming lessons; weddings; play dates for dogs; music videos; and photo shoots for swimwear.

One woman books Weinberger’s pool at his house in Long Island four days a week to swim laps for an hour.

Across Pennsylvania and New Jersey, about 500 pools are available through the platform, and more than 1,000 are on the wait list pending background and other checks. In Philadelphia, about 50 pools are for rent and 150 are pending.

California has seen the most growth on Swimply, but the state was tied until recently with the New Jersey-Pennsylvania-New York tristate area. The company sees the greatest potential for growth in metropolitan areas such as Philadelphia that have a large gap in those with and without pools, but it has seen interest in every state. Swimply offers sample guidelines and relies on hosts to comply with local regulations.

Hosts say that beyond covering pool maintenance, rentals are helping to pay mortgages and college tuition, and supplementing retirements. Some are earning tens of thousands of dollars; others across the country have made six figures, Weinberger said. Swimply charges a 15% service fee for each booking.

Most hosts offer guests access to a toilet, whether that’s in a pool house, the hosts’ home, or portable restrooms. Some offer grills, fire pits, and other extras.

Weinberger had just bought his first home with a pool when he heard Laskin’s pitch for monetizing private swimming pools at a meetup for entrepreneurs a few years ago. Laskin started by renting his neighbor’s pool in Lakewood Township, Ocean County, and soon other families in the cul-de-sac were using her pool, too.

» READ MORE: Natural swimming spots near Philadelphia

The pair knocked on about 90 doors of properties with pools in the area in 2018. They got four homeowners on board and built a small website.

“Within weeks, we had hundreds of strangers swimming in strangers’ pools,” Weinberger said.

Now the company is profitable, and the team has grown to 65 people with offices in Los Angeles and San Francisco. The company transitioned from customer waivers and now offers liability and property insurance. Swimply soon plans to offer rentals of other private spaces, such as basketball and tennis courts, gyms, and music studios, under the name Joyspace.

Keara Barber, a 21-year-old swimming instructor, said Swimply helped save the business she started building a few years ago, KB’s Super Swimmers, while she works toward opening her own swim school. Renting space in commercial pools was too competitive and expensive, and she had too many students to use guest passes at her swim club. Through Swimply, she said, she found her “dream pool” in Bala Cynwyd.

“I use their pool every week for swim lessons, and it’s been incredible,” said Barber, who lives in Upper Darby and works full time as a deal support analyst at We Buy Any Car. She books the pool for hours on Sundays for students she doesn’t travel to see.

» READ MORE: We need city pools, and how to find a pool in Philly this summer | Elizabeth Wellington

Kevin Barry, president and founder of Ardmore-based Summit Swimming Pools Inc., said it makes sense that homeowners would see demand for their pools. Barry’s company designs, builds, and services residential and commercial pools, and during the pandemic, he said, he and his workers became “busier than we’ve ever been in our lives.”

“For construction, we’re booked out for two years,” he said.

» READ MORE: Demand for pools surges for second year amid canceled vacations and long days at home

The commercial swim clubs and summer camps the company works with also have never been busier, he said.

On top of high demand, materials such as PVC pipe, steel, and pool equipment are in short supply due to manufacturing disruptions and rising shipping prices. Summit Swimming Pools has been waiting on three orders of hot tubs for more than a year.

In addition to supply challenges, the cost of building a pool can thwart some homeowners. Installation of inground pools can cost up to $40,000 depending on the features, while aboveground pools are more affordable for many people, said Michael Dean, a pool builder for more than 20 years based in North Carolina and the operator of PoolResearch.com, which gives advice to pool owners.

He advises owners to clean pools regularly, maintain good water circulation, ensure proper pool chemistry by testing the chemical balance often, and to “shock” pools with sanitizer regularly.

As for renting out backyard pools, “I’m not sure I’d do it personally, but I get the appeal,” Dean said. “There seems to be a very voracious market for this.”