It is the current flow that gives the shock/tingle, not the voltage. The PD (that's potential difference not PikeyDave) required to give said shock depends on the resistance of the path to earth. Obviously if you are wet and holding on to a 'live' A-frame with one hand whilst the other is in the briny then your resistance to earth is going to be much lower than if you were dry and not touching the sea. Normally you require a high voltage to get a shock as the resistance is too high for a meaningful current to flow. Bear in mind 9v is enough to give a tingle as anyone who has tested a PP3 battery on their tongue will attest
. Over 30mA is deemed dangerous hence why household RCD's usually trip at 30mA. The old party trick of static electricity in a balloon which makes your hair stand up is many thousands of volts, but there is insufficient 'charge' and therefore current available to electrocute you (otherwise there'd be a lot of demised kids entertainers and children!).
Also, if there is any coil or transformer that is causing the stray current then the shock is greater if it is switching. The induced reverse voltage as a coil is switched off is very high, hence why coils are used for ignition. It could be an ignition fault due to moisture in the engine or a damaged HT lead giving this stray current.