The reason I'm leaning towards a glycogen storage problem is precisely because of the "moving underwater" feeling which would suggest to me an energy metabolism problem. You clearly have had the same intuition, although your answer has been to blame lipid metabolism.
You might actually have type II glycogen storage disease, because that can manifest later in life, not necessarily since birth.
If you do have glycogen storage disease, and depending upon the variant there are diagnostic tests and therapies available.
There is an association for folks suffering from various forms of this pathology. I've linked their website here. Their site quite comprehensive and comprehensible.