O'Neill Men's Thermo X Short Sleeve Insulative Top

This really works. Keeps you warm both underwater and out of the water, while being breathable and fitting like any rashguard. Not quite as warm as say 2mm neoprene but close, and its breathable, thus comfortable in and out of the water. Can be worn under neoprene wetsuits as well, for an added layer of warmth. But I've used it mostly to stay warm at the beach for body surfing etc when the temperatures aren't quite high enough to be comfortable in the water without some form of insulation, but not cold enough to require the inconvenience of a shorty wetsuit. Due to its breathability, this would also be a great safety undergarment for cold weather boating/fishing activities. Its warm and breathable and if you accidentally fall in it would provide a decent amount of insulation to help slow the onset of hypothermia.

I don't know what kind of voodoo magical material this is, but I'm glad I found it. It's like wearing a very lightweight sweatshirt in the water. Typical rash shirts are nylon and cold when you put them on, cold when you get out and the wind hits you, and can be heavy even after you've wrung the water out of them. THIS feels like a warm cloud when you first put it on and keeps me warmer than a standard rash shirt when swimming in the pool. When you take it off afterward, it is really heavy and you think, "I'm gonna need a wagon to bring this home," until you wring it out, and then, as if by some Hogwarts magic, it's lightweight and feels almost dry. I'm 5'7"", 145 lbs and got a small. It's very tight but doesn't impede movement in the water. Like a fuzzy, warm extra skin. I might have to do something crazy like buy 10 more in case they stop making them.

Definitely my new favorite water top. The outside feels similar to most rash guard material, the inside feels like a cross between microfiber and soft felt. It comes together as softer and more stretchy than rash guards or neoprene. It does not form a water tight barrier like neoprene, but restricts flow and provides better warmth and cushion than rash guards. Because it is so flexible and the inside resists sliding across the skin, It is great to reduce chaffing under the arm or around the neck when warn under a wetsuit, PFD, or BC. I bought it for whitewater kayaking and its perfect, but I can tell I will also be using it for surfing and scuba diving, both as a stand alone top and worn under a wetsuit depending on the temp. Good job O'Neill!

Seems pretty decent and warm - haven’t swum with it yet but be sure to order a size that’s large enough. For me I ordered a 2XL when the size chart said L (I had to return the XL one size up as it was super tight).

I wore this to the beach last week. This rash guard has thick, soft fabric. Not only does it provide sun protection, it also protects against some nicks and friction against surf boards and boogie boards. AND... I was surprised that I felt warming wearing it in the water vs other more basic rash guard/athletic shirts in the ocean.I like it a lot!

This shirt is one of my favorite purchases that I’ve made in a long time. It is a total game changer if you are sensitive to cold water, even when swimming indoors in the winter time. I no longer have to give myself a peptalk to go to the pool. I do a deep water workout that is very vigorous, but because I have spine issues, the cold water can cause my muscles to seize up. Not anymore, however. This really really keeps me warm from the second I jump into the pool until I get out 50 minutes later. Some people complain that it is difficult to take off when it is wet. However, there is a method that works really well. First remove one arm, then remove the other arm and then lastly pull it over your head. If you try to pull it over your head when it’s wet without taking your arms out, it’s impossible, but this method that I use works very well.

I usually take an xl but bought an xxl based on reviews and could barely fit it over my head. Feels like I would really need an xxxxl but they didn’t offer more than just xxl. They should probably switch their marketing to focus on children’s clothes

To the point... The gym I go to has a heated pool but it's not warm enough to my liking. When my groin hits the water, I get an instant reverse hernia. It would take me 10 minutes to slowly adapt to the water temp. Using this shirt, I was pleasantly surprised how warm it kept my upper body! I can now go straight into the water and get swimming. I need this during winter months and it's great. Regarding size to buy, as others correctly suggested, get one size bigger than you normally wear.I'm 6', 185. I'm a solid Large in all polo/tee shirts. I bought an XL and it fits perfectly.The shirt is a little snug, as it should be. A large would have been too tight.I know this top costs a bit more than some others, but to me, it was totally worth it. I'm comfortable now, as in warm comfortable, going swimming is slightly cooler water.Highly recommend!