Maybe like, a close fitting suit of hollow, lightweight synthetic fiber. That would be both waterproof and warm. Or maybe one made out of stretch polyester blend, and it'll be ultra moisture wicking, traps warm vapor but keeps you dry, and it's also lightweight. Or perhaps synthetic fleece. That breaths well and is lightweight AND moisture wicking. Ooh, what about polyester blended with Merino wool? That would be warm and sorta lightweight. But that would be for land. As for if they want to stay as mermaids during the winter, perhaps a jacket (does the mermaid's tail need to stay wet? If not, then maybe a exact replica of their tails, only slightly bigger and maidmade of either of the materials above; synthetic fiber, stretch polyester blend, polyester blended with Merino wool, or synthetic fleece.) made of the materials above, which are synthetic fiber, stretch polyester blend, or synthetic fleece. I would also suggest the color of the suits be a dark color, because dark colors trap in heat more then light colors. So a black, dark grey, or another dark color would be practical.