I'm installing a 44" light bar and navigation lights on my duck boat, the light bar is 240w. What wire size would y'all recommend? I'll have the bar on one switch and the navigation lights on another, both being supplied by a DC battery.Thanks.

240 (watts) divided by 12 (volts) equals 20 (amps).

that will use a 12 gauge wire, 10 gauge would be better at that load, as 20 amps is maxed out for 12 gauge wire.

You will definitely want to use at least 10 gauge, assuming you are running the wire from the back of the boat to the front. Cast simplified things and didn't take resistance into account.

I always used extension cords for my light wiring on my bowfishing boat, for way more watts than what you are going to do. i've used cheap jumper cables as well, for extending trolling motor wires, which also work great. just a few options if you might have either laying around the house.

Don't do this lol

You could use a 12 or 10 gauge extension cord if you like, not a bad way to go but overkill for sure. As far as wire size and resistance is concerned, you're not running enough length to be concerned with the wire's native resistance. That said, run 10 gauge and put it in convoluted loom to protect it.