"Some immersion suits (as I have been taught to know them) do have an element of buoyancy at times, but that's due to inadvertently trapped air, insufficient to keep an adult afloat and usually distributed around the suit in a manner that does not safely keep the face out of water and severe windspray."
Many years ago I was asked to jump off of a low platform into a water tank at the RAF IAM at Farnborough for a 30 minute experiment which involved my wearing an immersion suit, but no LJ. Great fun for the first 5 minutes floating around on my back high out of the water, Then I became aware of subtle farting sounds intermittently coming from the wrist seals , firstly my lower extremities got deeper in the water and as the pressure in the suit went up , so did the wrist seal air losses and then the neck seal.
As you will imagine this process ran away and for the last 5 minutes I was desperately trying to tread water, a process increasingly hindered as the suit went in to creases as the air was forced out and became almost vice like in constraint regarding limb movement .
I was fished out as the 30 minutes came up, nearly exhausted.
The Aviation Medics explained that all this was in support of investigations in to the consequences of a couple of Lightning ejections over the North Sea. Incidentally "keeping the face out of the water" is of course why the greatest part of an LJ's air bladder is over the chest.