More than people expect, and it starts before you're properly at sea. Dolphins and penguins turn up as you leave the Beagle Channel. Out in the Drake it's seabirds: wandering and black-browed albatross following the ship, and petrels working the wake. On good days fin and humpback whales appear. There's usually a naturalist out on deck helping you tell the species apart.
Deck wildlife watching with a guide interpreting is one of the quieter pleasures of the crossing and it's the reason not to spend both days horizontal. A wandering albatross has the longest wingspan of any bird and watching one hold station off the stern without a wingbeat is worth being cold for.
Bring binoculars. The birds are close, the whales usually aren't.