Most likely bad ground.
On some vehicles the brake wire is grounded when not lit. Here's what happens case where you have a bad ground. 12 volts is on one side of the running light element in the bulb, there is no ground on the ground side of running light element (bad ground). The ground side of the running light element and the ground side of the brake light element are tied together in light fixture. Since the hot side of the brake element is grounded when not in use, ground is going through the brake element to ground the running light element. Both elements of the bulb are usually lit when the lights are on.
Remember that the ground is being supplied from the hot side of the brake element, well when the brake pedal is pushed, it removes the ground and put 12 volts on the brake element like it should, but now there isn't a ground for either element and all the lights go out.
Since there is no ground, I would be getting in the boat.