The most popular waterproof E-readers are Kindle Oasis 2(7" 1,000 mAh)and Kobo Aura One(7.8", 1200 mAh).
In both systems you don't have to buy books from their libraries you can sideload them easily. Kindle doesn't support Epub format but it is quite easy to convert epub books to .mobi and .azw3 using Calibre.
The battery life of the KO2 is about 15 hours at best. From what i know it is same for the Kobo Aura 1.
What i love about my Oasis and also the Voyage is the ability to use buttons to turn pages.
Here is a list of waterproof E-readers