Mermaids’ favorite food is Kelp, which satisfies much more hunger than even raw fish or human food. But buying from the grocery store can be expensive! You can harvest it from underwater plants, but where? When you’re a mermaid, you have level 10 diving skill so can go to any dive spot. I found some here at the cave at Pearl’s Deep. They are the small light-green plants (3 pictured below).
Each one can be harvested repeatedly, and you get 3-4 kelp with each pick! A great way to collect a good supply.