The question of ‘why am I so hungry after scuba diving,’ is down to two things. The first of which is probably down to dehydration. The brain can get confused and feel hungry when it’s mildly dehydrate. When in fact it needs more water instead. The second reason is that although scuba diving is a low impact sport, you still burn around 600 calories per hour. This is due to the extra heat loss underwater, due to water conducting heat 25 times more efficiently than air.
Scuba diving burns calories mainly due to heat loss as water dissipates heat from your body 25 times faster than air due to its greater thermal conductivity. Scuba diving also burns calories from low impact exercise in a weightless environment. The combination of both is up to 600 calories per hour.