put it on a differant sized lake--that way you can either be farther out or closer to shore------------------

being a boat, with a full lake of cool water--overheating shouldn't be a problem--however----as said--water pump-- what is the cooling system like--the automotive water pump or an external pump???? it could be seizing up from overheating--the coil could be bad--is it mounted vertical or sideways--if vertical, the internal oil could be low and coil shorting out when it gets hot--

so---most likely you have a temperature problem or a fuel supply problem------