It is hard to explain why a person who has just starved for days is able to be so robust in energy again.
But of course, this power came with a price, not long after the whole surviving crew sunk themselves into the winter Mediterranean, the chilling cold water immediately made them realise that puny state of their body. They are only pushing themselves on and on fuelled by the desire to survive and return to their Caesar, while the human's natural instinct to survive can provide them with a sudden strength, but that too can only last for a short while, and when this energy is depleted too, it shall be a bit of trouble for them.
Like now.