You don't say if you have had the boat for a while and it has recently developed this 'fault' or if you have recently acquired the boat and you have filled the header tank and it is 'losing' water.
If the latter, then the amount of cooling water in the whole system is quite a lot when you take into account the skin tank. engine block. calorifier etc and there could well be a lot of expansion taking place normally.
It might be that your header tank isn't large enough to deal with the expansion. Does it continue to lose water or does it reach a level and then remain at that level? Is so, then topping it up will just cause it to continually blow out the excess. But if it reaches, and remains at a lower level, then it might just be the header tank is too small for the system.