Many opportunities for improvement

Not bad, with potential. The Kobo H2O is indeed waterproof, floatable or not - even with the cover.
The cover lid has no positive locking. This means that the lid moves against the body and thus the Kobo corners are exposed and the protective effect may be canceled.
Immerse the Kobo into the water, the device must be taken out of the shell then, as no water flow is provided,
Perhaps all just little things, but in total a sign of little thinking along. On the special features the Kobo H2O is not received.
Own improvements eg by an additional rubber seal, small holes in the base of the casing and by a thin layer of foam enhance the shell significantly.