Cold water immersion suits are the gumby style and are thick neoprene. The Mustang work suit like you see the Coasties wear have closed cell foam sewn into them and offer more floatation as well as greater range of motion but don't have boots sewn in and they have a looser collar so you get more water intrusion and thereby less insulation. Like a wetsuit getting wet isn't the problem because you can heat that thin layer of water but you don't want the water cycling through your suit as that defeats any insulating properties you'd otherwise get.
There are many types of mustang suits. I've worn both the MAC 10's and the MAC 200's in Kachemak Bay. The MAC 10's are lighter duty, like you describe have no booties sewn in. They act like neoprene, in that they suck to your skin, and you are warm when wet. However, get out, start to dry out, and air gets against your skin and you freeze. The MAC 200's have dry booties, and a watertight neck seal. Feels like wearing a space suit.
Gumbys are for emergency donning. Suits like the MAC 10 are for wearing while doing an active job (Iv'e worn mine snowmachining). The MAC 200 is for doing a generally sedentary job while being prepared- like flying a helo.
