I recently bought myself some Pilot Kakuno fountain pens with extra-fine nibs. They were very cheap from Amazon, though they seem to have jacked the price up substantially since then.
The nib isn't very responsive, so getting a variable line isn't really very feasible, but it does flow nicely and the line is nice and thin.
I haven't yet tried out adding watercolour over the top of the line, so I don't know whether the Pilot ink is water resistant at all.