I assume you're going next year? I looked it up and next year the Spirit will be visiting Tracy Arm. I've never been but from what I read, there are times of the year you won't get too close to the glaciers there although the area is beautiful.
You of course can always visit the Mendenhall glacier in Juneau. I highly recommend that.
There are so many ships in Alaska in the summer I find it hard to believe that only this one ship meets your schedule. Check out the other lines and if you really want to see glaciers from the ship, go to Glacier bay even if you have to try and adjust your schedule forward or back a day.
You won't regret it.
Round trip Seattle sailings are the least scenic and have the roughest potential. Expect it.
Tracy Arm is not a given for glacier viewing. Probably only about 25%+ of the cruiseships make the full transit for glacier viewing, which is at the end of 30 miles in. Some lines are diverting more to Endicott Arm for glacier viewing of Dawes, but, my opinion only, on the "glacier scale" neither of these has the forte of glacier viewing. The transit of Tracy Arm is very very nice, Endicott- so so. Again, only my jaded opinion, having sailed all the glacier areas. Mendenhall, is OK, again, not real exciting and rare to see calving. But excellent for an easy access and nice hiking, good visitor center, and cheap.