Definitely! I could barely swim when I started mermaiding and am still not all that good. The technique used for mermaiding is called the dolphin kick, and it is fundamentally different than any normal stroke because your legs are tied together, so being a proficient swimmer won't help all that much anyway.
If you are new, though, I would recommend some things for safety, because the drowning risk is real.
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Start in a pool with a shallow end and progress from there. I learned in a hotel pool over a long weekend, and there is a pool rental site called Swimply where people rent out their backyard pools on an hourly rate.
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Buy a cheap tail, and practice with the monofin ONLY. Save the tail skin until after you've gotten comfortable. This is a safety thing. Cheaper tails come with cheaper monofins and they are easier to slip your feet out of if you're drowning. My tail is from Suntails and it cost about $80. FinFun will also do you well.
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Practice the movement on land first. A YouTube search for "dolphin kick" should get you some exercises
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If you feel unsafe, it's totally cool to wear the tail and just hang out near the water without going in! None of my photos are taken with me in the water. I consider the swimming and the modelling to be two equally valid ways to be a mermaid.

