But to answer the question, the skin doesn't lose it's water-proof qualities when you die. In fact, it retains it. The reason bodies bloat (regardless of whether the bodies were in the water or not) is due to a process known as putrefaction. This happens when the digestive enzymes inside your body 'go out of control' and eat away from the inside, producing gases such as methane and carbon dioxide that build up.
If you keep your hands in water long enough, you'll notice that they 'prune up'. In this state, skin is much weaker and tends to be cut up more easily. Now, being a dead corpse it would have been there for quite a long time and we can expect the skin to be much weaker so it would tear much more easily when someone pokes the corpse. Think of it as a weak balloon filled up to it's maximum capacity.