"Jij moet de hoeveelheid water verhogen."
Translation:You need to increase the quantity of water.
They're not quite synonyms in general. 'Raise' means to move upward, e.g. to raise something from the floor to a table. 'Increase' means to end up with more of something. In some (usually metaphorical) contexts, they have the same meaning - for instance, 'raising' or 'increasing' one's income would mean the same thing.