What kind of pen/brush do 水笔 and 签名笔 refer to? I was casually browsing Chinese artist blog, and the artist states that he/she uses 圆珠笔 签名笔 and 水笔 as medium for art.

I know what a ballpoint pen is. However, I am not sure what kind of pen is 签名笔. Is it a fountain ink pen?

Additionally, I am very confuse about the term 水笔, since I thought it refers to watercolor. However, when I search pictures on internet, only pen pictures appear.
圆珠笔 simply refers to ball pens. No questions there regardless of the dialect or personal usage preference. 

签名笔 could be referring to gel ink pen in common sense. But for artists I highly doubt that. A good possibility is technical pens. I am an architect myself and this is essential for anyone who wants a clean drawing. 
https://en.m.wikipedia.org/wiki/Technical_pen

水笔 is definitely not water color. It's a formal term for pen.