(This page contains information that I managed to gather between bouts of sea-sickness and being lulled to sleep on a rocking boat. So use the information at your own discretion: i.e. read on if you are a clueless tourist looking for reading material; go back if you are a PhD student looking for research material!)
The wildlife you are guaranteed to spot in any self-respecting cruise are the birds and the most common among them are the gulls - especially the Black Legged Kittiwakes and the Glaucous Gull (yes, it is relatively easy to tell them apart). You can see colonies of them occupying a vertical cliff. As everyone knows where their colony lies, every cruise boat stop in front of them. Gulls are pretty much all you will be able to spot on a short cruise. If you are on a day-long cruise at the National Parks, you are pretty much sure of sighting the next most abundant set of birds - Puffins, Common Murres, Bald Eagles and Guillemots.
Puffins (also known as the Sea Parrot) are the most watched and the most photographed. Being diving birds, they have quite dense bones making them quite clumsy fliers - invariably adding to their appeal. Around here, they come in two varieties - Horned and Tufted. You can tell between them by the presence (or lack of) yellow tufts. Common Murres (the deepest diving birds - they dive upto 600 ft) are the closest equivalent of Penguins that we have in the north of the equator. They are completely unrelated - only convergent evolution ensuring their similarity, having lived for years in similar environment. Their white bellies and black backs are due to the same reason - fishes cannot spot them approaching when looking up to them or down upon them.
Murrelets and Arctic Terns are a bit rarer to spot, but if you have a keen eye for the feather, you have a decent chance of seeing them. Among the animals, the easiest to spot are the Harbour Seals lazing on the floating ice near the glaciers, Stellar Sea Lions lazing on sea rocks and Sea Otters lazing on their backs right on the ocean waters. Of these, Sea Lions are getting increasingly difficult to spot in the Aleutians as their numbers have suddenly and dramatically dropped recently and no one has a slightest clue why. Humpbacks, Orcas and Porpoises are relatively rare, but if the season is right and cruise is long, you have fair chance of seeing all of them. Do note that spotting the whales is one thing and seeing them breach or even their tail flukes is another. Most of the whales would just blow pretty fountains and show their "humpbacks". You (or your captain, I should say) need to be patient to see the other tricks. Do note that if you see their tail flukes, you can stop tracking them. Tail flukes indicates a deeper dive and there is no knowing when or where they would surface.
Land mammals are harder to spot as they can hide easily in the bushes and they get easily scared off by a large group of people. But given enough luck and a long enough cruise you might be able to spot at least one of the four possibilities - Grizzlies, Black Bears, Mountain Goats and Moose.