Great by itself and even better under a wetsuit!
I got this 1mm top in a Small size to wear under my 5mm wetsuit while going through very cold water in slot canyons. It has worked great so far! I wear a wool shirt under it and it works even better! I've just worn the wool shirt and the 1mm top in cold water too and it does a great job keeping the water out and keeping my upper body warm. I have a very small neck and this fits perfect! It has stretched a little from my head, but still keeps out water and doesn't choke me. the waist is nice and snug too to help keep the water out. I wish the cuffs were a little tighter because that is the only place water gets in, but it's not too bad. I just wish they had the green in a Small! This is also reversible!
I have the 2011 version so it has more blue. :)