A car type switch would be suitable for most inboard engines although most of them now incorporate a steering lock and may be difficult to fit in a boat dashboard. However, if your engine is a 2-stroke outboard with magneto igniton then you need a switch that closes its contacts when switched OFF. This is the opposite to a normal ignition switch as it is used to short out the magneto to stop the engine.
Thanks guys for the advise, I went down to the boat today with a multi metre and found that it was the bloody loom conector block at fault, must be the damp over the winter, Thanks again Tony