The answers that mention that people on land cook with boiling water neglect the point, brought up by @OmarL, that water conducts heat very well. When you cook pasta in boiling water on the stove, it works well because the pot is surrounded by air that insulates the pot and allows it to be much hotter than the environment. To cook underwater efficiently, you'd want to build ovens with thick walls of insulating materials. Rock and sand would probably be your best bet. Your goal would be to reduce as much a possible convective transfer of heat between the water in the oven and the ocean outside the oven.
Now, the question is how to heat the ovens. Hydrothermal vents are definitely a good solution @TheSquare-CubeLaw are a good solution. Maybe you could pipe hot water through insulated (mostly rock?) pipes to merfolk houses.
But maybe hydrothermal vents wouldn't be very convenient or nearby merfolk settlements (you have to live where the food sources are). Electrical power could work. We run electrical cables through water all the time, they just need to be properly insulated. But how would merfolk make wires? Now I'm trying to imagine mining of ore, metallurgy, and fabricating insulating materials underwater. Probably some native metals like gold and platinum can be found underwater and used without any processing. Apparently seabed mining ( https://www.nature.com/articles/d41586-019-02242-y ) is now a thing for land-based humans, although I bet much of the processing happens on land.
Most of human technology has been based on burning stuff in air as an energy source and I would guess merfolk would need to do that too. It's the easiest source of concentrated energy for a preindustrial society. So, I imagine that the merfolk would need beach-based or maybe floating facilities where they dry and burn seaweed they collect. They could then operate smelting facilities on rocky outcroppings or on the beach. They could probably build dams and turbines underwater to extract tidal power. Could they build an electric generator that operates underwater? I might guess it would be easier to put it above water or fill a cavity with air for its operation.
The need to operate facilities on beaches or further on land might cause interaction or conflict with land-based creatures.
Maybe they would make floating homes near the surface and just start a fire on a floating platform and place their food below. The oven could be thermally insulated with pumice obtained a nearby volcanic island. Maybe it would just be easier to cook on top of the floating platform?
Maybe they could build electrical transmission circuits by drilling long boreholes through rock in the seafloor and filling these boreholes with seawater. Seawater isn't as good of a conductor as the aluminum wire we used for power transmission, but could be good enough to transmit electricity from the tidal power or ocean current generating station to homes a few hundred meters away.
It's really interesting to think about how technology could develop underwater. Would there be too many engineering challenges, making the merfolk remain in the Stone Age? Or would they be able to find ingenious solutions to doing many things completely underwater? Or would they mostly make use of beach or floating facilities and use technology similar to us?