I feel like the Oasis 2 is probably similar in toughness to other Kindle readers, with the addition that it is waterproof as well. That being said, the screen is not Gorilla glass, and both the screen and the metal back are vulnerable to scratches, so a case is helpful. A case/cover can also enhance one's grip on the device as well (keep in mind its unusual shape).
I personally like this case
https://www.amazon.com/dp/B07BVPHXTW...v_ov_lig_dp_it. It uses a similar design to the official cover, which was withdrawn by Amazon. It is thin light, easy to remove/put on, and comes in a variety of colors/materials. Some feel that it does not stay on well enough, but I think this depends a lot on how you hold it. I personally have not had any problems with mine falling off unless I squeeze it from the sides when picking it up. It would not offer much protection from falls, but it definitely will protect it from scratches, and help keep dust off the screen. It attaches with a magnet, so if I feel like taking it off to read, I can do so very easily.