So immersion suits are no where near as good as wetsuit/undersuit combo?
An undersuit goes with a
drysuit. Not a wetsuit. Drysuits are fully waterproof with neck and cuff seals and integrated boots. If used for diving they will have two valves fitted. One inlet to allow compressed air to be let into the suit, and a dump valve to let it out. Lightweight drysuits to be used in non-diving operations, i.e. flying, lack these valves. In other respects they are often identical, with the same seals, boots and usually a large thigh mounted pocket. They may however have a more robust boot fitted if requested. They are ALL booted and when used for diving an open heeled fin is worn over the boot and held in place with a strap around the heel. (Take note Backpacker).
Immersion or Flotation suits are bulky, Mr Blobby affairs which may or may not be entirely waterproof. Not suited to a cramped cockpit.