Hosteen,<br><br>First off, you are not exactly correct that traditional neoprene anti-exposure (exposure, immersion, poopie) suits are one size fits all. In fact, there are four standard sizes that fit a wide range of people from small to extra large. Custom suits can also be had. Having said that, they are even at best somewhat oversize since they are designed to be donned quickly in an emergency, over your regular clothing. SOLAS spec / U.S. Coast Guard approved suits incorporate a number of features that you won’t find in a conventional dry suit used for diving. Foremost among these is that via a pillow or other means they will support your head out of the water. Some are designed with full face covering, even standard designs have partial face protection. Both are important. They will support you prone in the water, not sure the dry suit will do that, and they are an enclosed “system” approach. <br><br>Typically, pilots using such suits wear them rolled down to the waist in flight. In an emergency the idea is you will finish donning the suit during any descent to the water. That usually works out, but may not be easy, especially if flying solo. In any case, they are still not comfortable to wear for long periods and are awkward to fly in. The dry suit you will need to wear the entire time and I suspect will become quite warm and uncomfortable, but that’s just an educated guess based on wearing other non-breathable suits for long periods. I have no idea how the authorities will view a regular dry suit instead of a proper exposure suit. That would be pretty much strictly an issue with the Canadian authorities whose rules must be followed when departing from their territory for North Atlantic flights.<br><br>A down side of any of the neoprene exposure/dry suits is that they could inhibit your egress from a ditched aircraft due to their inherent buoyancy.<br><br>The best solution, though hardly the least expensive, are the modern aviation exposure suit designs (Multifabs, Mustang, etc.) that incorporate Goretex fabric, providing a dry suit that breaths. They do generally require auxiliary insulation (long underwear, etc.), unlike neoprene foam they are not self-insulating, with a couple exceptions. They also require separate floatation, an inflatable life vest. As such, they won’t create a problem during egress.<br>