I use NRS 3mm paddling gloves when I'm in the ocean fishing from my kayak in cold conditions. They're warm enough that I can ab dive with them too. This is in water as cold as 40 degrees, for up to two hours without my hands getting cold. Because immersion in water pulls so much more heat than air, I'd be willing to bet that they're way more than enough needed for fishing in temps to 20 degrees.
http://www.nrs.com/product/24401/nrs-paddlers-glovesBut, I'm not fly fishing with them - cast and spin fishing only. However, if the weather is a bit warmer, then I just use the gloves sold at the marine supply place in Kodiak. I think they're meant to keep your hands from getting beat up while cleaning fish, but they take a little of the edge off the cold, and really help my thumbs from getting beat up while lipping rockfish. I don't have a link or know their name, but they have grey palms and a black mesh back. Very lightweight, but enough protection to keep hands warm while constantly wet. The good thing is that they're crazy cheap - like $5 a pair if I recall correctly.
P.S. Just dug them out of my fishing bag - they're Atlas 370s