Some use an external power supply (aka 'a brick'). Others have the power supply internally but use only 12v from it.

The tricky part about using 12v directly is that some devices are VERY picky about it. And since the voltage in a boat is likely to fluctuate a lot (from lows of 10v to highs of 14v) it's hard on the electronics. In some cases it may damage them. So ones designed for operation in a vehicle are definitely worth considering over something you'd have to hack. There are 12v to 12v voltage regulators, but I've not had the occasion to use one. These smooth out the fluctuations and provide stable 12v. But by the time you go this route you might as well just be using an inverter.

If you are trying to run directly off 12v dc circuit you need to know the minimum voltage needed to run the television.

In the past I had a 5.5" analog color television set that was for true RV use. It ran down to 11.8 volts dc. I later purchased a 9" analog color TV. While it stated to run either 120v ac, or 12v dc, it was not exactly true. It needed a minimum of 12.6 volts to operate. Since a fully charged battery is 12.7 volts, you would get about 1/2 hour operation before the battery fell below 12.6 volts. The television was designed to be used while driving (plugged into one of the 12v outlets of the vehicle) with the alternator keeping 12v circuit voltage above 13.2 volts, not directly off a battery while camping.

With the cheap cost of portable inverters (<$50.), it is simpler to convert 12 volt dc to 120v AC to power a television. The inverter regulates the power to fully utilize the battery. I have been using the same Vector inverter for over ten years.