I ordered two 20L bags from different companies. I like the Piscifun one better.
Pros: -- Good quality material, feels much more durable than the competitor bag. -- Nice handle on top, the padded straps are great. -- Better capacity than the competitor bag. Not sure how 20L is a different size in different brands, but this bag holds more. -- Cord and pouch on the front is nice for items you need quickly that don't have to stay dry. Competitor bag did not have this feature, so to grab a bottle of sunblock or water I have to risk getting everything in the bag wet. This is a thoughtful little "extra" feature that is really helpful.
Con: -- Straps are very long, like strangely long. -- I ordered black, and the bag now has some white/grey spots on it. I will be beating on this pretty hard over the next few years, so the aesthetics don't bother me, but be aware that it might show some wear with heavy use. -- Not sure it's totally water proof, I'll test further when I have time.
Neutral: -- This bag is roughly twice the price of the competitor bag, but I think it is worth the difference.
My 20L holds a towel, change of clothing complete with sturdy shoes and personal toiletries (comb, deodorant, cleaning wipes forfreshening up), large blanket, wallet/phone/keys and small first aid kit with room to spare. I ordered a 10L size to hold some items for day trips, like a pair of beach shoes, towel, sunscreen, camera etc. I'm using this for sailing and general travel.