Water materialization is a special ability found only among the Rainlords.
Normally, materialization powers are limited to a single element. However, the Rainlords and Sandlords have a single power each that breaks that rule. For the Rainlords, that power is the ability to materialize water. Aside from the fact that it affects a compound and is only found in Rainlord bloodlines, the power of water materialization seems to behave exactly like any other servant power.
For a long time, the powers of rain and sand were considered to be proof of a divine right to rule. A Rainlord who developed the power would be expected to be wise and a natural leader whether they were or not, and would be assumed to be more powerful than their age would suggest. This sometimes led to the lucky servant developing egomania to reflect the artificially heightened respect of their peers.