If
you have set the QNH your altitude will be correct at the airfield.
Most of the questions ask what will happen if you cross a mountain, or if you
are at a FL. They are above your reference point (the airfield) so if it is
colder than ISA your true altitude will be less than your indicated.
If you are flying below the airfield elevation the correction will be reversed.
In cold air, true altitude (the sea) will be more than your
indicated. That means when your altimeter reads zero you will still be some height
above the water.
Now the dangerous one is high temperatures. You will hit the water while the
altimeter is showing you some height above zero.
The altimeter assumes ISA because it is calibrated according ISA so if the
temperature is not ISA then the altimeter will falsely calculate the sea level.
If the airfield is above MSL and it is colder than ISA the altimeter thinks sea
level is higher than it really is.
That is why it is only correct at airfield elevation, and you have to correct
if you are above or below the airfield elevation.