Is it possible to charge a laptop with a DC-DC converter using a 12V deep cycle battery

You can if your laptop is using the traditional DC input. Many of the newer ones require intelligent at the power supply end and you'd better use an inverter for that (12V DC to 110V AC) like Solar Mike suggested.

If your laptop uses the traditional DC input, you'll have to note the followings:

The deep cycle battery shall have enough capacity to charge the laptop. For example, the 13" Macbook Pro has a battery of ~6Ah, if you used a fully charged 20Ah battery, then it shall be able to charge the laptop twice.

Note the discharge characteristics of the deep cycle battery and your DC-DC converter must be able to maintain the voltage and current required for the laptop's operation. Therefore, it depends on how much energy your laptop will require during charging or operation, and if your DC-DC converter can support that much energy without blowing itself up.

You'd also need a very robust output protection circuitry in your DC-DC converter so that it won't accidentally damage your laptop by, for example, over-voltage.

The output of the DC-DC converter shall be very clean and free of high-frequency noise, with very low ripple even when delivering at the maximum. Laptops, should have their own protection but it is never too much to ensure the cleanliness of the energy delivered to it. Think of it as the food you take.

Laptops might also suddenly draw a large amount of energy, such as when it switch on the GPU, make sure your DC-DC converter's output response could delivery what the laptop needed and deliver fast enough, and without much of a ripple in the output voltage. Or you may experience occasional hangs or data loss.

Last but not least, make sure the DC-DC converter's output is nice and friendly when the laptop switch on or off. It should provide a boring smooth but quick start and stop of the output.

Maybe.

At one time, a laptop charger simply supplied a fixed voltage. It varied a bit between manufacturers, but was usually about 19V.

Now some laptops use USB-C to charge. To get the correct voltage for the battery to charge, the laptop must communicate with the charger over the USB data lines to request the desired voltage. A simple DC-DC converter won't support this.

Some manufacturers, such as Dell, have added a data line between the charger and the laptop. On power-up, the laptop will communicate with the charger to check that it is compatible. If the charger doesn't respond, the laptop won't charge, even if the correct voltage is present.

It would be worth seeing if the laptop manufacturer offers a 12V car charger for their laptops.