Pretty much any waterproof ereader should be okay for you, or almost at least. The only thing is that waterproof e-readers are rare and hard to obtain.
Kobo offers H2O and Aura One. The latter is most easily obtainable currently, but it's big and needs a hyperexpensive cover for protection. The H2O comes in two editions, the older of which has a microSD slot.
Pocketbook offers Aqua 2. All is great with it, except the screen is comparatively worse than on Kobo H2O and there's no microSD slot.
And Tolino Vision 4 HD is also waterproof. No microSD.
A longer list
https://blog.the-ebook-reader.com/20...tly-available/