A hybrid sealion from a cross between the California sealion (Zalophus californianus) and the South American sealion (Otaria byronia) has also been reported. There has also been a genetically confirmed harp seal x hooded seal hybrid.

Among the marine mammals, a wolphin (whale/dolphin) hybrid occurred in captivity in 1985 where a female bottlenose dolphin and a male false killer whale shared a pool. The Wholphin's size, colour and shape are intermediate between the parent species. Named Kekaimalu, she has 66 teeth - intermediate between a Bottlenose (88 teeth) and False Killer Whale (44 teeth). The Wholphin proved fertile when she gave birth to a calf sired by a Bottlenose Dolphin; the calf was three quarters dolphin/one quarter Whale and thus looks more like a dolphin. Despite being fertile, Kekaimalu did not mother the calf (this is not uncommon in captive dolphins and was probably not related to her being a hybrid), but it was successfully hand-reared. In April 2005, it was announced that the same wholphin had produced another calf in December 2004. This second calf was also sired by a Bottlenose dolphin and at 6 months old was already the size of a 1 year old Bottlenose. The False Killer Whale is not a whale at all, but is a type of dolphin. Herds of False Killer Whales and Bottlenose Dolphins associate together in the wild and there are unsubstantiated tales of natural hybrids between the two species.

In 1933, three strange dolphins were beached off the Irish coast; these appeared to be hybrids between Risso's Dolphin and the Bottlenose Dolphin. This mating has since been repeated in captivity and a hybrid calf was born. In captivity, a Bottlenose Dolphin and a Rough-Toothed Dolphin produced hybrid offspring. In the wild, Spinner Dolphins have sometimes hybridised with Spotted Dolphins and Bottlenose Dolphins. In the wild, bands of males of one dolphin species have been observed to gang rape a female Spinner. Blue Whales, Fin Whales and Humpback Whales all hybridise in the wild. Dall's Porpoises and Harbour Porpoises have hybridized in the wild. There has also been a reported hybrid between a beluga and a narwhal.

Clymene dolphins, discovered in the 1800s, but not recognized as a species until 1981 are an example of a hybrid species. The Clymene dolphin originated as a hybrid between the Spinner dolphin and Striped dolphin.

There is an article "Two Presumed Interspecific Hybrids in the Genus Stenella (Delphinidae) in the Tropical West Atlantic," Aquatic Mammals 2005, describing two "presumed" hybrids observed in a single school of dolphins. Thesew were not hybrids of the same two species however. There are several articles about a hybrid between a melon-headed whale and a rough-toothed dolphin spotted in the wild near Kauai, Hawaii. There have also been a number of narwhal-beluga hybrids and possibly a specimen in a Danish museum.

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