There are several things that should be considered before you drink cold water immediately after meals. To call them a danger are something of an overstatement, however they can be quite uncomfortable and eve painful in some cases, and so should be avoided. The main problems tend to be that cold water can change the state of things that you have eaten, and can react with them to make them harder to digest, which can cause indigestion.
When we eat things which are liquid when hot but will solidify when cooled down, such as butter or some cheeses. Drinking cold water immediately afterwords can cause them to become solid again in the stomach, and become harder to digest. Normally when we eat these things the temperature in the stomach is enough to keep them in at least a semi liquid start long enough for them to be broken down. This then means that they can soon leave the stomach and pass through the rest of the digestive system to be converted into energy.
If the foods we have eaten solidify again in the stomach then they take longer to digest and can bring about indigestion and even acid reflux in some cases. Having a lot more water than normal in the stomach also means that the stomach acids used to break down the foods that we eat are diluted, and so take much longer than normal to work. This can leave you feeling tired and sluggish after you have eaten, which can tend to slow you down for a few hours.
Drinking very cold water can also cause pain to those with sensitive teeth, or who suffer from frequent headaches or migraines. Migraines in particular can sometimes be brought about by consuming something too cold for the body to comfortably process. Drinking a lot of water after a meal can also cause you to feel very bloated and can cause stomach aches. This is because some food expands when immersed in water. This then stretches the stomach and causes us pain. This is prolonged as well because a high water presence means that the stomach takes longer to break down its contents.
In terms of evolution the human stomach is designed to consume food at similar temperatures to our own body temperature. This means that things that are extremely hot or cold means extra work for them to be digested, Similarly when we eat very spicy food, it is a lot harder to break down correctly and in some people can cause diarrhea. This is because most people aren't used to eating them. this is similar to the strain that the body is put under when we eat things that are very hot or cold temperature wise as well.
The best thing to do is to drink with a meal rather then immediately after it. This also means we will tend to eat less food because soem of the space in the stomach is used up by the water. And drinking room temperature rather then very cold water is better for digestion as well.