Sometime around when long sleeve season started, I developed eczema which has since spread all over my arms. (Yes, I’m seeing a doctor for it.) The past week, I’ve found out that my eczema really, really doesn’t like sunblock… from both an aesthetic (“ewwwwww, that person has really contagious looking rash!”) and a health perspective (to prevent it from worsening) I think I’ve got to wear long sleeves that effectively covers the eczema as well as blocks out sun. What is the coolest long sleeve piece you’ve found? 
CoolMax. They also make light weight clothing with a SPF rating- try REI, Sportsmans Guide or Sierra Trading post.
The coolest long sleeve thing I have is this shirt from Coolibar. It’s a light opaque material with lots of strategic mesh and a fold up collar and fold out back-of-hand covers. Even so, it does get kinda hot sometimes (although it’s defintely cooler than my old stand-by of a long sleeved t-shirt). I mostly wear it walking around at all day regattas with only a jog bra underneath; I haven’t worn while exercising because I generally try to go out when the UV index is low (like mornings or evenings).
I have this shirt: http://www.columbia.com/product.aspx?p=4339&cat=2&top=2
It is very lightweight and has a UPF of 30. I don’t fish, but it has a cute little rod holder tab that I’m trying to figure out another use for…
Try a sporting goods store with a wide selection of workout clothing. The most popular brand right now is UnderArmour but you can get similar stuff from Nike, Adidas, Hind, etc.
It’s all synthetic stuff that wicks moisture away and you can get it tight or loose fitting.
Without a doubt, the Mountain Hardwear Wicked T:
http://www.altrec.com/shop/detail/10640/#photo
I have two of the short sleaves and one of the long. My favorite running shirts. If you don’t believe me, check out the reviews.
The stuff from American Apparel is a very lightweight cotton.
Thanks! I’ve followed up with a couple of recommendations on this thread. We’ll see what wins out.