Even a leisurely swim can burn upwards of 400 calories an hour, more than double the amount of walking.
The comparative low impact of water activities in contrast to running make them perfect outlets for those nursing minor injuries, as well as the elderly.
And it's not just short-term gains, there's also lasting benefits to swimming.
Calm waters
While the physical boosts of swimming are widely documented, the mental health benefits of getting into the water are less well-known, yet equally as impactful.
Open water swimming in particular -- with its naturally colder temperatures -- is increasingly understood to have mental health benefits.
For those willing to brave the chill, the feel-good hormone dopamine is released by getting into cold water, ensuring an endorphin rush that can last hours after drying off.
Just being in a so-called "blue environment," close to the ocean or a body of water, is known to lower stress responses.
"My first thought as I dove under the surface of the water was that I felt a little more buoyant than usual, likely due to the added pounds brought on by quarantine," Lieber said.
"But as I continued to glide through the water, my initial concern about weight gain was replaced by a feeling of catharsis, as though the water was cleansing me of the stress that had accumulated during the coronavirus pandemic.
"Stroke after stroke, I could feel my mood lifting, my mind clearing and my body loosening."
'Epiphany moment'
Based in the UK, Mental Health Swims is a volunteer-led peer support community that organizes open water meets up and down the UK.
Having received her mental health diagnosis in 2018, Ashe initially took up running but lost confidence after some frightening slips on ice during the winter.
By the close of the year, she was feeling "really unwell" and "everything was challenging," yet on New Year's Day, Ashe -- quite literally -- dove into a new future.
Braving the 'Loony Dook' -- an annual event that sees fearless participants take to the freezing waters near Edinburgh, Scotland -- Ashe returned to the beach shivering but changed.
Six months later, 30 people joined Ashe for a swim meet and the group's growth has been exponential ever since, even through the pandemic.
This year, Mental Health Swims will host over 80 swim meets -- from Cornwall in the southwest of England all the way up to Loch Lomond in Scotland -- led by trained volunteer swim hosts with an emphasis on inclusion and peer support.
Reasons for joining vary. For some, it's the sense of community, while others search for mindfulness and that post-swim endorphin rush.
Ashe loves the water as an alternative safe space from the more intimidating environment of the gym, a passion that has breathed new life into her mental health.
"I have learned that my differences are a strength rather than something to be ashamed of," Ashe said. "I never thought I could do the things I do today.
"I will always have a mental illness, but I am much better at looking after myself these days. I still have big feelings, but with medication, therapy, outdoor swimming and healthy, happy relationships, I am doing really well."
'Revitalized'
Few are better suited to speak to the physical and mental health benefits of swimming than Sarah Waters, who lives in the coastal county of Cornwall.
Diagnosed with rheumatoid arthritis during her time at university, Waters has lived with symptoms of the chronic inflammatory disease for over a decade.
Aggressive treatments and medication proved massively draining, and after returning from traveling and working in Australia, a lump on her neck turned out to be skin cancer.
The physical and emotional toll of operations to remove the cancer and shifting treatments was compounded by the need to shield during the pandemic, but Waters' fortunes turned a corner when -- after a little nudge from her mother -- she took up sea swimming.
"She started going and she kept saying, 'You've gotta come in, it really does help with your mental health,'" Waters told CNN.
"When you get out, you get a bit of a rush, almost like you've been awakened in a way. I know that sounds really weird, but it definitely does give you that tingly feeling that you've achieved something that you never thought you would be able to do before."
And so began a dogged commitment, even through winter, to swimming two to three times a week -- at times, Waters' only way of leaving the house due to shielding requirements.
For Waters, these physical boosts dovetail with the mental health benefits.
"You always do get the fear feeling, just before going in like, 'Can you do it?'" Waters said.
"But I do it and then afterwards it's a sense of achievement in a way, for your physical and mental well-being, it definitely does something.
"With all the meds, you can feel quite fatigued a lot of the time -- when you've got a day off, you're just so tired that you don't feel like you've got the energy to do it - - but once you've done it, it does revitalize you.
"Once you start improving your symptoms of anxiety or depression, it can physically give you benefits as well."
After finishing his first swim in over a year, Dr. Lieber looked ahead to the start of a four-night stretch working in the hospital's intensive care unit.
"I usually dread the first of these night shifts," he said. "But somehow the task seemed more manageable than usual.
"Whatever happens tonight, happens. No matter what, there will always be tomorrow."