Might be memory-fade on my part (not unknown!) but as a spotty yoof on 'E' Dispersal at Scampton from late '64 to early '66, I do not recall such luxuries as 'plunge baths', heated or not. Best I can cudgel up are life-ex life rafts filled with water at ambient temp. A tad difficult to 'plunge' into in the depths of a Lincolnshire winter...