The only reference that I could come up with for “water soluble” is an article by Mercola at http://articles.mercola.com/sites/articles/archive/2012/02/23/oral-vitamin-d-mistake.aspx.
Most of the article is based on making sure if you are going to take Vitamin D as a supplement is to take D3 which is closer to what humans make when they go out into the sunshine and not D2 which nowadays still comes is a prescription form. Vitamin D is fat soluble meaning it stores in your fat for future use which is a good thing. A supplement or sunshine can do this. As a side note though you will not store until you reach levels of 40ng/ml.
As for water soluble, the article later talks about the work of Dr. Stephanie Seneff. (I cannot comment. I have no knowledge on her work.) What she is really talking about is when Vitamin D3 is made by sunshine it gets carried through the body a little differently than the supplement Vitamin D. From what I can remember this is true but the end result seems to be the same.
So sunshine is good for you and the other UV rays it contains other than UVB may do things for us that we have yet to discover. (Actually some have been discovered already!)
Hope that clarifies things.