Amazon has popularized waterproof ereaders, but they are far from the first company to release one. Over the past 5 years quite a few ereaders have been released by a bunch of companies.
The first official release of a waterproof ereader came from Pocketbook, followed by Kobo, Barnes & Noble, and finally Amazon in October 2017.
Here's a list of ereaders that are currently available in various markets around the world.
Edit: The list was updated in June 2019.
Amazon's flagship ereader, the Kindle Oasis 2, was the first Kindle to have a 7" screen and it was also the first Kindle to be released with waterproofing. It has an IPx8 rating, meaning it is certified for up to 60 minutes immersion in up to 2 meters of water.
This Taiwanese ereader company makes several waterproof ereaders, including a couple models for Kobo and the previous Nook model.
B&N's new ereader, the Glowlight Plus, is also waterproof. That device has a 7.8" screen and retails for $199 on B&N's website. It has an IPx7 rating, meaning it is certified for up to 30 minutes immersion in up to 1 meter of water.
Kobo also carries several waterproof ereaders made by Netronix. The Kobo Aura One features a 7.8" screen. It has waterproof guts that give it an IPx8 rating, meaning it is certified for up to 60 minutes immersion in up to 2 meters of water. (This device was discontinued.)
The Kobo Aura H2O 2 has a smaller 6.8" screen and the same IPx8 rating as the Aura One. Kobo also used to sell the Aura H2O. It had a waterproof shell, but is no longer available.
And finally, the Kobo Aura Forma has the same 7.8" screen and IPx8 rating as the Aura One, only with a one-handed design. It costs $299 on Kobo's website.
The Pocketbook Aqua 2 is the second waterproof model from this Ukrainian ereader maker. It has an IP57 rating and can handle being submerged in up to 1 meter of water for up to 30 minutes.
The Touch HD 3 is a 6" ereader with a 300 ppi screen, a color-changing frontlight, and a retail price of $159. It has an IPx7 rating, meaning it is certified for up to 30 minutes immersion in up to 1 meter of water.
Tolino has 2 waterproof ereaders, including the 7.8" Epic and the 6" Vision 4HD. Both devices have waterproof guts covered in HZO, giving them an IPx7 rating equating to protection for 30 minutes of immersion at a depth of 1 meter.