I'm utterly bewildered. I know that I'm unqualified, but surly in all cases of some form of dizziness upon rising, there is a distinct tendency to grey out. He says that his vision is fine.
This guy is describing symptoms that are not at all like a momentary fall in BP, and he adds other parameters that could indicate a quite different kind of problem. There may be some red-herrings with the sting and the (presumably) hot sunshine during a swim at this latitude...but who knows.
As one who has extraordinarily low BP, I know that when I go on a crash diet, I will, after about the third day, get quite severe symptoms when rising suddenly. Using anti-g stomach muscle techniques, I can immediately restore things to normal. His symptoms are quite different.
He uses the word clumsy...and combined with the bad headache during the swim, I would feel better if this guy was in the doctor's surgery right now.