It's not uncommon to feel so wiped out by the third day of a dive vacation that you miss dives to take naps and don't really feel like pulling on that wetsuit again. You'll kick yourself after you get home. But right now, you'd rather flop into a hammock.
Heat loss is probably the biggest energy thief.
Its effect is pretty obvious in cold water, where two dives a day is enough for many. What's surprising is that warm tropical water can actually take more heat out of you in a day than colder water can. Fooled by its apparent warmth, you don't protect yourself as much and you stay in longer. Swimming in scuba gear seems almost effortless because of your apparent weightlessness. But water is 800 times denser than air, creating 800 times more resistance to every movement you make.