Preventing devil fruit users from swimming, if i could change only one concept in the series it would be that. Reasons:
1. OP's world is extremely diverse and incredibly rich yet not fully utilized, it's basically consists of the Blue Sea and one small continent. So There are many locations underwater the strawhats could have discovered. Bubbles are shit, i want Luffy to dive in, find treasures, new devil fruits in the bottom of the ocean, like a real pirate.
2. Without the sea weakness Zoan class devil fruits will get a shitload of sea creatures. I dislike the idea that there isn't any fish Zoan tbh, we need sharks, whales and a mythical Kraken. More importantly, Ship battles. No sea water weakness means devil fruit users can battle it out on sea without hesitation since they won't be drowning. Some devil fruit attacks would even look much better underwater.