Swimming usually fucking sucks in games. It's just walking, but slow and with an extra axis. Rarely does it add much even in modern games. In GTA5, every time I get in the water I feel like I'm wasting my time and even the water vehicles kinda aren't that interesting. The main value in it compared to Vice City for example is that you don't die when you go in the ocean, with the implication being that you should swim out right away so you can go fast.
>old games with swimming
Super Mario Bros, but that's not open world. More recently, Donkey Kong Country, which also isn't open world, to be fair. You could also stretch the definition of swimming to include games like Pokemon, even if it calls it surfing instead. I bet some old roguelikes had open world swimming but I'm not into the genre enough to know.