Rob - a some of them can be shot straight out of the bottle, Vallejo Model Air, Color of Eagles and some MM Acryl paints to name a few. Others like Tamiya need thinning.
As far as thinning with water, I know Vallejo, Color of Eagles and I believe Gunze can be thinned with water, while MM, Boyds and Tamiya have an alcohol based thinner (you can even use 91% isopropyl from the drug store to thin them. Can't comment on the others though...