Because at 100ft above water you only feel the air pressure there. While at 100ft below water, you feel both the air pressure ( strictly speaking in physics ) plus the water pressure there.
Also, note that water is more dense than air. This means for each space, water exerts more pressure compared to air for that same displacement of space.
Further, pressure that we feel is created due to the displacement of water or air molecules respectively by the object. So, neighboring air/water molecules try to “displace out” the object that caused the “imbalance” where originally there were only air/water molecules before the placement of the object.