We took this day cruise after taking the train to Seward from Anchorage, and it afforded some of the most majestic views and experiences you could ever imagine.
First of all, I doubt you'll get to pick your captain, but ours, Captain Mike Sutherland, was the best navigator and narrator / story teller you could have imagined. (although I imagine they're all amazing) He was so knowledgeable not just about the nature, but about the history of many things we saw. It was also awesome that as people (myself included) stood on the bow of the ship and spotted marine mammals, he would steer the boat so we could get better glimses (always being aware to keep a safe and respectful distance).
First things first though. Bring a coat. And a relatively heavy one. We went in late June and when you're traveling across the open water at a good clip, it can be dang cold outside. And that's where you'll want to be to get the best views. You can hang inside where it's warm, but if you want to truly see the best things (mountains, glaciers, and wildlife), you'll post yourself on the front of the boat. But wrap up, because it's cooooooold. Oh, and take binoculars and/or a good camera.
The best thing I can say for day trips from Kenai Fjords is to book early. This is something you don't want to miss. If you're not much of a boat person, the water can get a little choppy, but if you get fresh air outside and focus on the beautiful view, I'm confident you can make it through the 6 hours without much of a problem. Lunch is served on this boat and there's a snack bar if you need something else.
In summary, not sure what else but to say JUST DO IT. You really can't go wrong with any of the cruises, but to go see the beauty only accessible by boat and the wildlife and nature of Kenai Fjords NP is a must do.