It was stated on the show that dolphins don't drink anything.. This is presumably because they spend their lives in water and some wonderfully in-depth research indicated that they don't drink the water around them... however.....
Dolphins being mammals (classified by a couple of unique attributes, not the least of which being sweat and mammary glands), produce milk, and the Dolphin calves do indeed drink this milk throughout the early stages of their life.
An elf replies...
For a show that airs in the winter and records in summer, you might be surprised to hear that research on QI usually begins in January of every year. The elves get together once a week where we go through everything that we have learned in a week; this continues for a couple of months before we're ready to start turning the work into a script.
It is in these meetings that I get the chance to tell the rest of the team about the various quibbles we receive in a week. There's often a bit of a groan - especially if we've made a real howler - but never had a quibble elicited such a groan as this one.
Dolphins are unable to drink seawater as it is too salty. They get their water from the foods they eat - squid and fish and have no direct source of drinkable water. However Gareth's is a quite brilliant point - of course we'd missed the point that baby dolphins drink milk.
Dolphins can drink milk while their mother is on the move. It is squirted directly into the calf's mouth and the babies' tongues have a frilled margin to seal them to their mother. The milk has a low lactose content and contains up to ten times the fat and protein found in cows' milk, as a result it is much thicker and richer. It sounds like perfect milk for the lactose intolerant amongst us, but it does smell of fish and tastes particularly oily, perhaps not so great on your cornflakes.