There are some underwater electric beasties, in the sea. And, in a fantasy world... nature is always worse.
My thoughts and ponderances
So, let's say the mermaids do have rubber, for simplicity. Great... now how do they insulate themselves without suffocating their gills?
Could they build a Faraday cage around themselves, maybe...? That's tricky though, as wouldn't that require ferrous metal? And that rusts, until the invention of stainless steel.
I guess the Faraday cage will still work, even if it's rusty? Not sure if they could cover it with some non-insulative, yet waterproof material, so they don't have to go about wearing a rusty cage.
TLDR: I figure a mixture of a faraday cage and a rubber suit is a possibility. Could it work?
Feel free to name your own ideas of how mermaids could protect themselves from electricity.