Guadalupe fur seal, Islas San Benito, Arctocephalus townsendi
In addition to Guadalupe fur seals (A. townsendi) I have dived among Northern and Galapagos fur seals (Callorhinus ursinus, A. galapagoensis) and have seen both species adopt this head-down posture as well, and suspect that it may be a natural resting position for fur seals that share their waters with large sharks.
Guadalupe fur seal, Islas San Benito. San Benito Islands (Islas San Benito), Baja California, Mexico