Gentoo penguins are widely reported as the fastest underwater swimmers among penguins, with a commonly cited top speed near 36 kilometres per hour. The number circulates in reference works and zoo material, and gentoos are certainly quick, propelled by stiff flipper strokes and a dense, low-drag body.
The qualification matters. Dive loggers attached to foraging gentoos record typical travel speeds far below that headline, in the range of a few kilometres per hour, because sustained swimming at high speed is metabolically expensive. Peak speeds are short bursts, used in pursuit or in escape, and the classic figure rests on a small number of observations rather than a large logged dataset.
Gentoos are also the shallow-water specialists of their genus, taking crustaceans and fish on dives that are usually well under a hundred metres, though considerably deeper dives have been recorded. The open question is less about the top speed than about what gentoos gain from it, since their prey does not obviously demand extreme pursuit velocity.