Good grief, we're not back to the old swimming argument, are we?
In 3.x, everyone can swim, since the Swim skill can be used untrained; the basic DC is fairly low - DC 10 for calm water - and you can take 10 on a Swim check.
In 4E, everyone can also swim. Same rules apply, except you get better (every even level) at swimming, along with all other physical activities.
So, neither system is good at modelling someone who cannot swim at all. Let's move on...
Humans have an inborn instinct for swimming, just like all other animals. We just need to overcome our fear of water/drowning (which doesn't take much time), and the basic motions of swimming come naturally. Sure, practicing to be a world-class swimmer is not the same as paddling along in the water, but that is not the issue (since D&D is not about setting athletics records).
Incidentally, I am personally really overweight and out of shape. I spent last summer going to the gym thrice a week for 2 hours (a lot less exercise than a typical adventurer gets). Guess what? Even though I spent no time practicing swimming at all, when I went to the pool in the fall, I was much, much better at swimming than I was in spring prior to the gym. So, practicing one type of physical activity can obviously make you better at other physical activities.
The only thing I will support here is that 4E's 1/2 level bonus is too good. But some sort of bonus needs to be there, for both gamist reasons and verisimilitude.