To a first approximation, air pressure at altitude X above sea level would be the same, but altitude X above sea level would be higher in absolute terms. read more
The atmospheric pressure at sea level will decrease by an infinitesimal amount as the sea level rises. A device called a barometer is used to measure atmospheric pressure. The pressure is caused by the weight of the air above the barometer at sea level. Imagine that the sea level rose by some significant amount. read more
The deeper you go under the sea, the greater the pressure of the water pushing down on you. For every 33 feet (10.06 meters) you go down, the pressure increases by 14.5 psi. Many animals that live in the sea have no trouble at all with high pressure. read more