I am a little confused. First this post is in the wrong section. Should be in the water block construction section. Second the title says water?
Anyway, .5mm fins would take a good mill with little excess play. Certainly possible though. The outer sloped wass can be done with a ball endmill. Everything else is pretty much standard square endmills. Won't be easy. If you get the base right and actually get water flowing through all those channels you wil havea decent block even without the middle inlet.