"Jens K" wrote
So now the question is this: could I simply move my car's battery to
the boat? Let's say for a four days cruise during summer time. Will it
still start the car afterwards? I guess there will be the pleasant
side-effect that the car will be less likely to be stolen.

It'd be like leaving your car's interior lights on for four days. Would the
car still start? Probably, but maybe not. (Mine didn't, leaving me stuck).
Would the battery's life be shortened? Probably, but maybe not.