Kaztuz is correct in pointing out that vomiting should not be induced following ingestion of poison as it can cause more damage. This is especially important with a corrosive substance .
However, drinking lots of water to dilute the poison is a procedure that is not advisable.
To begin with, the key to preventing harm to the body following the ingestion of a poison is two-fold, namely dilution and neutralisation. Whilst the water acts as the dilutent, it may not be the correct neutralising agent for the poison and this is where A&E departments come in. They have the experience to provide the correct antidote. Diluting a poison may reduce it's toxicity temporarily or not at all, but it does not neutralise its effect.
Another reason is that some poisons could react exothermically if water was sent down behind them. Whilst the stomach does contain enzymes, water, hydrochloric acid and other substances, these would not prevent a potientally catastrophic exothermic reaction when more water was added. The effect on the gastrointestinal tract would be devastating.
Another reason is that we have to remember that not all poisons are soluble in water. I'm thinking of the non-polar organic solvents such as chloroform and benzene as well as substances such as petrol. As these are all insoluble in water, the ingestion of water would serve no purpose. The same would apply following ingestion of toxic inorganic insoluble salts such as copper(i) cyanide, a popular chemical used in electroplating.
It's best to get the patient to A&E urgently with the bottle or container.