Sorry, I would hate to hijack this thread.
If you intend to connect to the DC jack on the board you want a power supply of ideally 7V - 9V. 12V is OK if you're just running the Arduino from it.
What's the rationale behind the warning not to use a 12volt (13.2 Battery) DC power supply? I understand that the Arduino has a Linear Regulator which might dissipate a lot of heat, but anything other than that?
Should I add a Switch-mode in between?
Now, add to the equation the problem of 6 bilge motors running from the same power rail. Any added problems? As far as I think, no, seeing as each motor has a flyback diode in line, and the regulator the Arduino Due uses has a pretty nice ripple regulation looking at their datasheet.
But correct me if i'm wrong.