They already have them. They are called "survival suits" or "immersion suits".

I have looked into immersion suits and the literature indicates bulky coveralls with permanant insulation with no indication of an inflation scheme. It seems to me that a lightweight system involving an inflation scheme might be easier to wear and more boyant than the ones currently indicated

In a garment such as coveralls, the buoyant material needed to keep a person afloat would be relatively thin, as thin as 1/4," and could take the form of foam insulation that doesn't absorb water. The garment needn't look much different than traditional foul-weather gear. Keeping the person warm while immersed in near-freezing water is a much more challenging problem.

immersion suits and survival suits are available however they aren't comfortable or easy to move in if they are going to do the job. also for a new product to become available there is a huge amount of red tape and standards that you need to do which pushes the price up.

I have just been looking at launching a product in this category as its a related industry but the end product would cost £400+ mainly due to the EN standards that are required.

great idea but not practical in the real world im afraid, but if you don't want one safety stamped they can be made for less than £100!