Water proof extra dimensional spaces

[tlotig]

So I am looking for extra dimensional spaces for my undine to use.
Are Handy Haversacks waterproof? If I dive into water will my backpack fill up?
Pathfinder's pouch and smuggler's wayfinder are two options, any others?
I think there was a magic item that gave you a pouch on your chest but I can't find it

[Maezer]

I think in general you want to assume it take an act of will to move something to or from an extra dimensional space. No one wants to be measuring air pressure to determine if an extra-dimensional space is overloaded.