I'm quite sure that when people ask for swimming what they want is for their character to be able to move on the surface of deep water with a particular animation (swimming). We don't need swimming physics for that. Our characters already move by bascially sliding across the terrain. The only thing needed to add swimming is to 1) Make transitions from different on-land movements into the swimming animation. Jumping into the water, running into the water, walking into the water. 2) Offset the character model so it 'sinks' below the water level. 3) A swimming animation. That's all. Then it's just back to the sliding across the terrain.

Also, I'm quite sure that people asking for swimming aren't asking for maps to be redesigned to allow swimming. Just to have the option to swim in those places that have accesible water. Places with lakes or coasts, like the very tempting Costa del Sol one, or the huge water body outside Camp Bronze Lake. Right now all that water is only off the map because there are invisible walls around them. And the reason those invisible walls exist in the first place is to not bother with swimming. It's a lazy design choice. The 'redesign' of such areas to allow people to transition from water to ground movement and vice versa would be minimal. And in some cases (like in Costa del Sol) it would be non-existent.