O'Neill Men's Thermo-X Long Sleeve Crew-Neck Rashguard Top, UPF 50+ Sun Protection with 4-Way Stretch Thermal Fleece Fabric

This is perfect! I can’t wait to dive with it, I typically dive in the tropics April-September and sometimes get chilly during the 2nd dive of the day (specially in the spring/fall months). This shirt is lined with a fleece like material, I suspect this will be the perfect amount of extra heat (without it being too much) that I need to be comfortable. The fit is tight, but that’s what you need, there should be no wrinkles in the fabric - that’s being said, the measurement chart is spot on!

Used in the Caribbean during May snorkeling adventures. Perfect weight. Keeps you warm, but is light and comfortable to wear. Highly recommend. (Size Large fit perfect for 6’2” @ 185 pounds.)

I wear this under my swimsuit tank top (I'm a woman) to keep my upper body warm in the winter months while doing water aerobics and water walking in a heated chlorinated pool. So far it is holding up just fine - I have worn it during 24 1-hr sessions and it has not faded, has not stretched, nor is it showing any signs of breaking down. It keeps my upper body and arms comfortably warm. Even though the Thermo-X fabric is quite stretchy, these tops run very small. I am a fairly large woman (usually wear Woman's 2X) and needed to buy a men's 2XL. Because it is a men's, it is a bit snug at the hips, but the rest of it fits well. Fair warning: once wet it is a bit hard to take off, but once you get one arm out of a sleeve, getting the rest of it off is pretty easy.

I’m not allergic to the cold, but in water cooler than the Caribbean I prefer at least a little insulation. We took a trip to Hawaii for the first time and enjoyed snorkeling, and I’m glad I got this to do it in. Fits well, not too loose but not a death grip either (get one size up from your normal tee shirt size), sleeves just long enough to fit my long arms and long enough in the torso it doesn’t ride up like a crop top but not so long it comes down past your pockets. The fleece microfiber lining or whatever it is soaks up water and keeps enough of it against your skin to be warmed by your body and not get too cold too quickly. It’s not a wetsuit of course, actually holding that water against you continuously and not letting it out, but it does seem to help. It definitely doesn’t make it too warm, either, striking a nice balance in temperate waters.

This shirt exceeded my expectations. I swim laps outdoors and it's gotten cold. This shirt makes it lot more comfortable on cold or rainy days at the pool. Like another reviewer suggested, I ordered one size larger than I typically wear. It is snug so the water doesn't pump too much and it is a little difficult to get off but it works well. I'm about 145 pounds, 5'-9'' and I ordered a large.

Own a bunch of O’Neill rashguards so naturally picked up a Thermo X for fall/winter. It’s basically what I would call a quarter mil rashguard as it’s slightly bulkier than their standard rashguards given the slightly thicker material with fleece lining. It’s thinner than 0.5mm tops though so 0.25mm seems like a fair description. No issue with movement restrictions. I wear this under a union suit that is under my dry suit for kayaking. Have been down in 20s on the water and not slightly bit cold whatsoever. Works exactly like I hoped. I have some thermal compression shirts I bought for this particular use case and picked this up afterwards. While the compression shirts are warm they sit a tad looser, nothing crazy but noticeably different fit as this sits tighter on body. But the Thermo X is definitely warmer, sometimes paying up is well worth it, this is one of those times.

Good fit in general for a thin built man but loose at the neck (mine is 14+ and I ordered Small). A lot of water would rush in this gap so I wear a thin rash shirt above. It's warm and so far after about a month in a chlorine pool it seems durable

I've never bought a rash guard or insulative top before, but we just bought a new house with a big pool and it's just too cold outside of the summer months and too expensive to heat. I'm a bigger guy and the XL fit great. It's thick and spongy but not heavy. I had no idea what to expect. It does not constrict any movement. I kind of look cool wearing it, and even gives me a little slimming look, if I do say so myself. I can easily get into a cool pool and at first I feel just a bit of the coldness, but in 15 or 20 seconds, its great! I spend 30 to 60 minutes in the pool after 5PM and it never loses the insulating effect. It's easy to get off and it dries super quick, just letting it hang over the balcony. I'm not sending a picture, because I don't think anyone wants to see that, BUT I for sure recommend this. Saw ones that were TONS more expensive. Don't know how much better those are, but like I said this one works GREAT!!!

Works great, I bought this specifically for snorkelling in Hawaii, I get cold fast no matter the water temperature and this kept me toasty.

works to keep you warm in cold water, true to size, appreciate the extra length since other brands pull up