I go to Seward every year (for the past fifteen years) and take at least three boat trips every time I visit Seward. The national park cruise, that takes a minimum of six hours, travels to one active glacier and spends twenty to thirty minutes in front of the glacier to observe calving. Since the boat travels out of the bay and spends more time travelling, you may see more wildlife. I have seen humpback whales, orcas, Stellar sea lions, Dall porpoises, and other marine mammals on these trips.
I also enjoy the wildlife cruises that remain within Resurection Bay because they focus on wildlfe, travel slower, and spend more time with animals. You will NOT see tidewater glaciers that calve into the ocean. You will see puffins, Stellar sea lions, sea otters, and bald eagles. You have a good possibility to see Dall porpoises, humpback whales, and even orcas. I have also seen mountain goats, bear, and even a porcupine. Nothing is guaranteed. If you do see humpback whales, you will probably see dorsal fins and flukes. I have seen breaches (I saw my first breach in Alaska four years ago and have seen at least one every year since then) but the whales were fairly far away from the boat. If you really want to see breaching humpbacks, go to Maui or Monterey Bay. Humpbacks are feeding in Alaska and spend most of their time below the surface. To see orcas and Dall porpoises, you have to be in the right place at the right time. The sea lions and sea birds tend to congregate in specific areas, so they are easy to see.
The longer cruises that go to the glaciers do spend a brief time in rougher water, so if you are prone to sea sickness, you could get queasy. The wildlife cruise is in calmer water.