2 very different causes being discussed here: "dizziness" means different things to different people- hence why it is a "heart sink" syptom to most octors.
Cold water effect on ears- as per caloric stimulaton test- likley to casue "true vertigo"- feeling of actual spinnig, accompanied by jerking eye movemets, - like getting off a spinning roundabout at he fair- ear plugs might help.
Postural hypotension- low blood pressure on standing- caused by vasodilation/ loss of compressive effect on wetsuit on legs/ abdomen. Could also, I supoose, be a heart rhythm problem- ever worn a HRM at these moments? Causes light-headed feeling, like being about to faint, may get "greying out" of vision. Need to sit/ lie down 'til it passes, and then get up slowly.