@Kateness I had a vague idea I'd seen a mermaid in a boat and I finally remembered when and what happened. It was in my Isla Paradiso Bunch video when Haley met up with Salty in Mermaid Grotto and he invited her to the Mermaid Secret Island. They both went up to the surface and Salty jumped into the boat with Haley. When they got to the Island Salty reset and Haley had to wait for him to swim all the way from wherever he'd reset to. It took a while. It's in the video but I don't remember which one it was in the first set of videos while they were still trying for the Isla Paradiso Badges. In the end they didn't get the badges and I suspect it was due to the household having more than one sim in it.
But other than that I've never seen a mermaid do anything other than swim. It's super annoying. If I'm playing with mermaids as the main thing I tend often put them on the fast Ultra Speed to get to where they're going. But they often take all day to get there. Having had that complain they are fun to have and very pretty if you've got a few with different coloured tails. I tend to have them living close to where I am most likely to want them to swim to. But it still takes a long time for them to get where they are going.