I highly recommend getting the best, all around wet suit you can, and then wear a 1 mm neoprene rash guard underneath when needed :-) Save $$, and adds versatility.

Blueseventy have the thermal reaction that has an extra thermal layer. https://www.blueseventy.com/collections/thermal

Im sure other brands have something similar.

I swim in cold water with a regular wetsuit (orca sonar), a lycra rashguard and a trisuit underneath, neoprene socks, and neoprene scuba hood. Arms, legs and torso always feel fine; most problems come from head and face, if really cold

I've a huub thermal. Aside from being warmer, it's no more or less restrictive than the non thermal version of the same.

The Blueseventy is supposed to be warmer, but it's also more $$$. The Huub is same price as their normal AEGIS suit - upper mid range $.

In reality it's still head, and toes that limit the time in water. Wearing a 1 piece trisuit which mutes that rush of cold water at the base of your spine does a lot. However for me, it's the burning on my face and the 'ice cream' burn on my forehead that is the limiter.


Edit to add. I grew up in the north of England, and learnt to swim in the north sea on family day trips to Whitby and Scarbrough in the 1970s. And so I think that the above needs to be considered in the context that as a 6 foot 4 inch 93kg guy, I have always been an embarrassment to my yorkshire heritage. I now live in the warm bit of new zealand where the water temperature goes down to a winter low of 16 degrees and I still used to get borderline hypothermia and need to get out after 30 minutes. I am a huge pussy.

Someone correct me if I'm wrong, but regardless of whether or not a wetsuit is marketed as 'warm', it doesn't change the fact that the max neoprene you can have is 5mm.
So I would assume the warmest wetsuit is the one that uses 5mm for the most areas on it.
Check out Desoto, I've always found it plenty warm.

I have a DeSoto Waterrover for sale if you are interested. It is not legal for any USAT or ironman events, but it is quite warm. It is also the fastest wetsuit ever made and is the reason why they made the 5mm rule. Let me know if you are interested, and I can send you size info.
Stephen J

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I believe my local reality has been violated.
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Happiness = Results / (Expectations)^2

"thermal" wetsuits possibly have some kind of lining, besides neoprene

Interesting. Do you recommend a sleeved or a sleeveless one? Which brand?

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Next races on the schedule: none at the moment

My go to wetsuit for cold water swims (62 and down) is the Aquaman Gold Cell. You can get them much cheaper than retail price if you watch for sales. Below is from their website. I'm not affiliated with them in any way. Based on reviews I've read on Slowtwitch, this is also one of the fastest wetsuits.

Aquaman Cell Gold– Almost Zero Water Retention

The Gold Cell is a high end model with high performance capability and will keep you warmer. This model is made with the Metal Cell technology, which is neoprene on the outside and inside, treated with SCS to make it very slick when in direct contact with the skin and the water. The end result is that the neoprene stays very close to the skin and allows virtually no water into the suit. This, in turn, forces your body to heat less water, thereby keeping you warmer. The suit is made with two layers of GIGA #40 from Yamamoto giving it a very good quality/price ratio. One layer lines the inside of the suit to keep a closer fit and one layer on the outside. This model is a perfect suit for a triathlete who is looking for performance and warmth. You will love the flexibility, comfort, lightweight feel, fast removal, and warmth in cold water thanks to the Metal Cell technology. The shoulders of the Cell Gold are made of GIGA #40 to allow maximum flexibility for totally free movement. The forearms have the “Aero Design Propeller” to provide a better catch of the water during the pulling phase of the stroke. The collar, made of GIGA #40 has a new “Revolving Collar” which positions the front of the collar forward by 20 degrees and is 2cm lower allowing for more comfort and reducing the entry of water through the collar area. Another feature is the Flash System on the back of the neck to prevent chafing and aide in the opening of the zipper. Like most Aquaman models, the zipper on the back opens from bottom to top.
Bigger Velcro behind the neck for easier closure.
Hips roll control – Aquaman has added a panel of Rubber Dome to the hips to help flotation and balance when you are on your side.

I've seen the B70 Thermal & there is a fleecy like inside. Pretty nice/plush, spoke with others owning it & worth the $$. **Disclaimer: I use B70 & am a huge fan of their stuff for quality. I've owned ROKA stuff too & it just doesn't stack up to the quality of B70 on anything I've owned from them.

This. The Huub is same wetsuit with fleecy warming lining. The 'rules' are about the thickness of the neoprene.