The largest operator out of Seward is Kenai Fjords tours. They've been in the business for years. Renown Charters is using a new catamaran this year from Seward and Major Marine also does a fine job (they're known for the special buffet meals on board). Mariah charters focuses on a smaller boat with a more personal experience. All do an excellent job

The half-day trip focuses on Resurrection Bay and never really explores Kenai Fjords National Park. They often hug the shore looking for wildlife (eagles, puffins, otters, sometimes orca and whales). Kenai Fjords Tours has some half-day trips that take you to Fox Island for a meal.

If you really want the true experience, go for the longer trips that go to Holgate or Aialik Glacier and the Chiswell Islands. Spectacular scenery and
usually plenty of wildlife (never any guarantees up here). Word of warning - the journey into the open waters of the Gulf of Alaska is often known for rolling waves and boat motion. It's really exciting for some but stomach churning for other. Motion sickness pills taken one hour before your cruise departs is highly recommended. If you wait and take it after the cruise has departed, you are often too late.

Also remember the weather - layered clothing and rain gear can come in handy on these cruises.

There are also several cruise operators out of Whittier which is (A) closer to Anchorage and (B) run through the calmer, protected waters of western Prince William Sound. These can include trips to Blackstone Bay (shorter tour), College Fjord, Esther Passage, and Barry Arm. I think you see more glaciers on the Prince William Sound tours although the Seward trips tend to have more wildlife.

As an extra bonus, the Alaska Railroad provides rail service to both Whittier and Seward that connects with the cruises. The route along Turnagain Arm is really scenic (beluga whales and sheep are often spotted). You can also drive to both if you wish but the train saves dealing with traffic.

Both Seward and Whittier are highly recommended and we usually do both during the summer. For what it is worth, the temperatures the past few days in Anchorage have been in the 60s and 70s with long hours of daylight.
Enjoy your visit!