The Arduino can be powered by the USB cable (convenient during programming), but for most applications this is not a good solution, because you don’t want to leave a computer connected all the time.
7-12V can be fed to the main power jack, but this is useless for marine/automotive systems where battery voltage is almost always >12V.
Average power draw is only 50mA, and there are several ways to supply it, but for me the best one is to use an external regulator chip to reduce the boat’s “12V” down to exactly 5.00V, the true input requirement (this will bypass the Arduino’s on-board regulator). The external regulator output is wired directly to the Arduino’s 5V and GND pins as shown below. LM317 is a fine option. Another great one is here, but it does require a smaller screwdriver and more care to adjust the output to 5V. Don’t forget that step, with either of these options! I have also used Pololu D24V22F5- more expensive but no adjustment required.
Also buy several power supplies when ordering- they’re cheap and good to have a few around in case of mistakes.
