If you 'search for' someone or something, the need to find it is far more serious than if you were 'looking for' it. In your example, if you were thirsty at home, you 'look for a drink'. You are thirsty, but the need is not serious or great. But if you were in a desert, dying of thirst, you would more likely say you are 'searching for' something to drink. The situation is far more serious and you *must* find something to drink. Another example: If you went to a park and lost sight of your child, initially you 'look for' him. You are not very worried - he no doubt just stepped out of sight. Now you can't find him and begin to worry. You now start 'searching for' him. It is far more serious and necessary that you find your child.