Body Fat %
Due to being extremely athletic in the past, (and probably due to a relatively healthy diet), my body has maintained a very low body fat percentage even after many years of barely exercising.
(roughly 7 - 10%)
This has few significant disadvantages. Low body fat results in less insulation and causing you to tolerate cold environments less. Thus even just leaving the swimming pool would cause you to shiver a lot.
It's also really hard to build muscle.
(Your body will burn your muscles for energy whilst exercising if there's no fat available)