The body of water, known by Victoria residents as "The Gorge," is a narrow tidal inlet between Craigflower Bridge and the Selkirk trestle.
It connects Victoria Harbour to Portage Inlet.
Historically, much like Howe Sound, it was heavily polluted by industry and sewage.
The Gorge Swim Fest Society's mission is to "promote and enable swimming in the Gorge Waterway by supporting initiatives that sustain the health, awareness, and vitality of The Gorge."
Rhymer grew up in Victoria and her mother is still there.
The idea for the swim came when she last visited Victoria and spent time with a family friend, Henry Leong, who is involved with the Gorge Swim Fest Society.
"We realized that we both had a common interest in swimming, so he took me to a few of his favourite swimming spots along The Gorge," said Rhymer.
"When I was a kid growing up in Victoria, The Gorge didn't have the reputation of being a suitable swimming spot at all; it was known for being polluted."
Now it is a great swim spot.
Pre-COVID, the Gorge Swim Fest was hosted there each year and it's one of the "go-to" spots for triathletes training, and it even sees a handful of dedicated winter dippers.
"I really love how the community came together to make this place special and beautiful again, and thought it would be an interesting challenge to try to swim it from end-to-end, Portage Inlet to Victoria Harbour. And why not support one of these fantastic local organizations in the process?"
Rhymer has launched an online fundraising campaign alongside her swim.
Money raised through the campaign, "Swimming the Gorge for GSFS," on GoFundMe, will donate 100% of the proceeds to the GSFS, she said.
Founder of the Swim Wild Squamish outdoor swim club, Rhymer swims year-round in the open water.
"In the summer, I can train longer distances when the water is warmer, and in the winter, I can acclimatize my body and mind to stand the cold water," she said.
She recently completed the 12-kilometre Skaha Lake swim from Penticton to Okanagan Falls.
The Gorge swim is about half that distance but more challenging in other ways.
The Tillicum Narrows will likely be the trickiest part of the swim, she said.
"At this location, the entire Gorge Waterway squeezes through a mere 45-foot-wide channel. As the tides go in and out, this can mean some significant rapids, standing waves, and strong currents. In fact, these rapids can go one way during the flood tide and the other way during the ebb tide, all in a single day."
She will attempt this swim at slack tide, when the water is going neither in nor out.
"At this special time, the currents should be minimal and hopefully, the Narrows will allow me safe passage," she said, adding safety is her primary priority.
She will get out and walk around if the conditions are not favourable for a safe swim.
"Otherwise, I'll plan to complete the entire swim without touching the boat, land, or any other floatation devices while wearing only typical pool swimming gear: swimsuit, cap, goggles, earplugs."
She said her other concerns about completing her goal are boat traffic and seaweed.
She will have a kayaker alongside to help her navigate through the boat traffic.
"The seaweed isn't harmful of course, but it freaks me out when it touches me when I'm in the water," she said.
"It was probably one of the reasons why I didn't pick up swimming until much later in life," she added. "I was scared of things like seaweed. For this swim, I'll try to stick to the deeper parts of the waterway where it will be less weedy, but I may just have to breathe, stay calm, and swim through it."
Rhymer is watching the tides to see when the very best day to do the swim will be this month.
Keep up with plans for the swim on the fundraising page.
Once the official date is set, she encourages Squamish folks to come out and cheer her on in Victoria.