Why can't Devil Fruit users swim? I've only seen bits and pieces of One Piece, but I know that Devil Fruit users (people who have eaten a Devil Fruit) can't swim. I never understood why, though. How does this work? It doesn't seem to give arm or leg disabilities, so what would keep them from being able to swim?
It is said that the Devil Fruits are the fruits of Sea Devil, so even if most of them provide a great power, they're cursed. Quoting One Piece's wiki:


  The sea rejects the new Devil Fruit user and they become helpless
  while submerged in water


Also, Oda said in SBS that Devil Fruit users are vulnerable to all types of water, not just salt/sea water. Basically any type of "standing" water (ocean, lakes, rivers, etc) will affect them, while "moving" water (rain, waves, etc) won't.

Overall, I'm not saying the problem is the sea or something inside the users, what I'm saying is that it's both. And as to why users are affected by Seastone/Sea-Prism Stone/Kairoseki, as stated by Smoker:


  It gives off a wavelength that is the same as the sea itself


Basically it's a mini solid piece of the sea so that's why Devil Fruit users are affected by that as well...