Kobo documents that the touchscreen won’t work properly when wet so I think you’ll need to rethink your plans to read it in the shower.
https://help.kobo.com/hc/en-us/artic...-Kobo-eReaders
The waterproofing seems more aimed at ensuring it survives a dunking and will still work after being dried, not that you can use it while (say) scuba diving.
Audiobooks might be a more practical solution.