Here’s an excerpt: In submaximal conditions, at 95% of a maximum speed over a 549-m swim, Bassett et al. (1) showed that drafting affects the metabolic response to swimming. Oxygen consumption was reduced by 8  +/- 12%, blood lactate concentration by 33 17%, and the rate of perceived exertion by 21 +/- 10%. The lower resistive body drag (passive drag) forces encountered by the swimmers at maximum speed are responsible for the observed metabolic change (5). These forces are 13–26% lower than those for the lead swimmer, depending on the velocity of the triathletes.

Whoa, doesn’t that make your fins flutter? Well, if you’re still awake then here’s the entire study in PDF format. But just remember, someone has to lead.