OK, just to set the record straight, here's a quick "Exposure Suit 101:"
There are primarily two types of exposure/immersion suits commonly used in General Aviation. The least expensive old-fashioned kind are SOLAS approved marine types made of inherently buoyant closed-cell neoprene foam designed for emergency use in the Commercial Maritime environment. They are very loose fitting to be able to be quickly donned over clothing For an example see:
https://www.stearnsinc.com/NC_ProductsList.aspx?selection=3&CategoryID=4504549 These are bulky and very hot to wear out of the water and, thus as noted, are typically worn with the top down around the waist and you finish donning them when needed in a ditching emergency (easier said than done, but it works, most of the time). The closed-cell foam also serves as the insulation. Being so bulky and inherently buoyant, they can present egress problems, as well as problems getting into a life raft, but they have saved many lives. I don't recommend these, but they do work.
The other style of exposure suit is a modern Gore-Tex exposure suit modeled on a traditional flight suit. Originally designed for the military, this is what fighter pilots wear flying over colder waters, what the USCG helo crews wear, etc. They are designed to be worn just like a flight suit and being Gore-Tex do breath and are reasonably comfortable. They do require you to wear additional insulation, which can be tailored to the temperature of water being flown over, all they do is keep you dry. They also require a life vest as they are not inherently buoyant. See:
http://www.multifabs-survival.co.uk/products.cfm or
http://www.switlik.com/pdf/u_zip_it_brochure.pdf . This is the type suit I recommend.
Class dismissed...