I have 3 of these that I wear every time I go into the water for any water sport all year long. The fabric is a thin, durable synthetic material - perfect for wearing in the water in any temperature - either alone or under neoprene. I wear it kayaking, scuba diving, and bodyboarding.
ABOUT THE SIZING: These are actually true-to-size for the intended purpose of the design. They are designed to be protective wear for your skin while surfing. If you are surfing on a surfboard or a boogieboard or if you are bodysurfing in large waves, you need your surf shirt to be a tight compression fit in order to prevent rash on your torso from the board. I am 5 ft 2 in and weigh 115 lbs. I wear a Small or an Extra Small for surfing. If you are not wearing a tight-fit synthetic shirt like this one, strong ocean waves push your shirt up your torso, leaving your skin exposed, resulting in abrasions from being thrashed against your board in the waves. HOWEVER, I also own this rashguard in a size Medium which I wear under my PFD when I kayak -- either with or without a layer of neoprene on top of this rashguard for warmth. When kayaking, my PFD and spray skirt keep the shirt from riding up if I swim, so the shirt doesn't need to be a compression fit. And when I take my PFD and spray skirt off, the size Medium is still form-fitting, but not so tight that every piece of cellulite shows as another reviewer mentioned happens when wearing the "correct" size.
This is the best material - next to neoprene - for water sports. So, if you will be wearing this for casual water sports, then go ahead and order 2 sizes up for comfort as many other reviewers have suggested because there is no better fabric. However, if you are wearing this in waves, follow the manufacturer's size chart.