I got the Smiling Case for use with 40mm S5 SS:
https://www.amazon.com/gp/product/B083R89RWJ/
Works well enough, and is not too hard to pop on and off. The color blends well enough with my black band.
However, if I swim with it, water gets inside the case and the screen becomes unresponsive to touch. I'm sure this is the case for any of these types of cases. I can take the case off for swims, but there is a lot of potential for scraping my watch against something when swimming (especially if my kid is with me in the pool).
I'm mostly worried about the screen, as I shattered mine when I dropped it because I was standing and adjusting the band. Since I have stainless steel, I'm not as worried about the sides of the watch.
I'm thinking of getting a screen protector that you put on with adhesive. The Sigen one looks good, but appears to be out of stock for the 40mm model. Another option would be to get a waterproof case that I'll use when swimming, but those are rather bulky, and would probably want to take it in and out of the case, which would be inconvenient.
This is my first Apple Watch, and it's possible I was just unusually unlucky when I dropped it, so maybe I should just try caseless, and hope I don't have to use my last remaining AC+ $75 screen replacement. I at least know to be very careful when putting on the watch or adjusting the band (rest my arm on furniture when I do that now). But I'm rather clumsy and always banging my arms into things, so I dunno.