How does water content affect magma?

The presence of molecular water decreases the melting point of the minerals in the magma.

In a silicic magma, the presence of water can also make it more explosive when the pressure on the magma is drastically reduced as the magma gets closer to the surface. Bubbles of steam are in the magma, and they pop when the air pressure in the bublbles is greater than the pressure of the surrounding magama.

Saying that magma will not melt without water demonstrates that some one does not know what he is talking about.

The presence of water in a rock alters the melting point of the stable minerals. Hydrated minerals tend to have lower melting points than water-free minerals of similar chemistry. The presence of water also affects the chemistry (elemental composition) of the magma produced during partial melting of a rock, both by impacting which minerals will melt and in what order, as well as by providing structural changes to the magma that enhance the solubility of some elements and decrease the solubility of other elements in the silicate liquid.

This is in part the reason that granitic magmas are so different both physically and chemically from basalts. Basalts have low water contents, higher temperatures of solidification, generally low viscosity, and generally silicate poor chemistry. Granites have relatively high water contents, viscous magmas, presence of volatiles and large ion lithophile elements, high silicate contents, and generally lower temperatures of solidification.

it just like this you make a thick milk shake and now add more milk to it. it will get thinner and thinner. just like this when water is added to magma it gets thinner.

OR

lets suppose if you are cooking meat in a pressure cooker while your are not adding water what will happen it never cook but burn. similar is the case with magma. you cannot melt a rock until add some water. in great depth if there is high temperature which can melt any thing but remmber there is high pressure which always not allowing temperature to melt that rock. so if there is any water or liquid is coming to that hot mass of rock it mean we have reduction in pressure. and so the rest of melting is done by temperature.

Here are some a sites that discuss it.