Expedition: general discussions.

Battles in the sands and cities, the enemy fleet was defeated off-screen a hundred years ago... why do you need a waterproof bag? Where are you going to swim? In a combat tactical bathtub on wheels?

Sooner or later, she would have to swim. That's one.
The option of fleeing from Calcite across the sea was also quite likely. That's two.
There could also be other reasons why Erika might end up in the water. That's three.