A large value capacitor as close to the PP power connections as possible may help; eg. 10,000uF or more, 25V or 40V rated - one with screw terminals rather than wires, as that will be more reliable in an environment with movement and vibration.
Connect a smaller value plastic film capacitor, eg. 0.1uF, across the large one as well. That can help absorb short "spikes" that big electrolytics don't handle well.
I'd guess there is not a lot of metalwork on a small boat, so grounds are also just wires, anywhere away from the engine etc?
Taking an extra ground directly back to the battery may help things?