I guess this is because you successfully press the jump key and you have a block under your feet that it works.

Nonetheless, it means that diving off water from a place where it is shallow (basically any shore) is potentially harmful to the player if a spell is equipped on the boots, with the danger increasing with the spell's cost.

For instance, my jump-dash spell will not propel me while getting out of there but drain my PSI quickly. Oddly enough, I lose more PSI looking down than upwards.