Please accept my apologies for the misleading information in my previous post.
With our next internal build, we will make the alignment of the watermark dependent on the TextAlignment property.
However, if you want to have the watermark and the text with different TextAlignment, you will have to edit the control template of RadTextBox and set the desired alignment to the PART_Watermark.
Let me know if you need additional assistance.
the Telerik team