Unless of course the mermaids can use "super" low frequency like our subs, but wouldn't the quality become poor. How can the mermaids enjoy 4K quality digital TV programmes much like us without laying cables all over the sea floor? They share similar technology with us in the 21st century CE.
Why would you worry about transmitting an electromagnetic signal. I don't even use that anymore myself. All our tv comes into our home through fiber-optic cable and in the end through shielded wires. Some of the wiring for TV transmission (and all other kinds of telecommunications) in fact runs under the ocean. Makes no matter whether the sea water is conducive or not. And you say about not laying cable all over the ocean floor. But, laying cable all over the landscape has not stopped man, why would it stop mer-people?
They can ultraviolet light broadcast TV.
Ultra low frequency radio and ultrasound both have bandwidth too small to stream 4k video signals. As you pointed out, higher frequency RF works poorly due to a high attenuation coefficient. They can use light, specifically ultraviolet light to increase the distance. Still, the range is terrible, they'll need repeaters about every 10 meters (or less).
They'd be better off using buried cables or they can use floating RF antennas with wired connections straight down to the TV (rather than "laying cables all over the sea floor").
There are heat dissipating hydrophobic silicon gels which can keep the water and salt away from your electric components. Some of the new cell phones can survive submersion, so why not a 4k TV. As for getting the signal to the television, I think you are stuck with cable if you want a quality signal.
It is not necessary to lay cables across the sea floor at all, it is only necessary to hang a cable from an antenna buoy on the surface and run it down to the underwater 4K TV. That way, atmospheric rules for radio propagation can be taken advantage of while still having a bandwidth that would be impractical underwater even with heavy multiplexing.
Alternatively, merpeople could use laser communication between antenna buoys and their seafloor buildings, taking advantage of water's blue-green transmissive frequency band, and achieve a high bandwidth that way without cables. However, we could expect frequent interruptions by floating debris unless lasers were multiplexed and separated.
Finally, if merpeople's structures were floating and anchored rather than on the sea floor (which might make sense for a mammalian, air-breathing species), then a rooftop antenna and a cable would be all that is needed.