Dan Thomas wrote:

Airplane altimeters read zero only if the airplane is taxying
into the ocean. Or flying deep in Death Valley or the Dead Sea Valley.
Zero isn't necessary for operators of subs or airplanes or anything
else. I can see that depth to keel would be safest, with reference to
depth charts. In aviation we use maps with obstruction and terrain
altitudes on them.

Perhaps you would like to look up QNH and QFE and get back
to us.

Rick