Cuvier's beaked whales hold the record for the longest breath-holding underwater, based on the longest dive ever recorded.
Scientists recorded a Cuvier's beaked whale diving and holding its breath for a total of 3 hours and 42 minutes. The record was recorded during a 5-year study of 23 members of the species.

Beaked whales in the study had an average dive time of 59 minutes. Only 5% of individuals observed had dive times exceeding 1 hour and 17.7 minutes.
Some other marine animals are capable of holding their breath for a long time, such as sperm whales for about 1.5 hours, whales and elephant seals are not far behind when holding their breath while diving for up to two hours.
Marine mammals can stay underwater for so long in part because their muscles are full of a protein called myoglobin, which stores oxygen and delivers it to muscle cells. Humans also have myoglobin, but at a much lower density.
According to a 2012 study, the myoglobin of deep-diving marine mammals is positively charged, causing proteins to repel each other, keeping them from sticking together and clogging muscles.
In addition to myoglobin, the researchers believe that Cuvier’s beaked whales’ long dive times are due to their low metabolic rates, meaning they don’t use up oxygen as quickly. The whales have a better tolerance for lactate (a byproduct of cellular metabolism) building up in their muscles when they need to switch to anaerobic respiration.