We booked our Icy Strait Point tour through the cruise line when we cruised Alaska with another line in 2016. We did hop off the ship and book a whale watching tour in Juneau, though, and that was one of the highlights of the cruise. We looked at those prices prior to cruising and we did get a discount booking that day on the dock (along with bus to and from Mendenhall Glacier), so it was worthwhile. There are a lot of ships and a lot of tour boat operators in Juneau, so I'm sure you could probably get a tour when you get in port there. I wouldn't bet on it in Icy Strait Point, as that port is a lot more limited.
Icy Strait Point is a very tiny port, purrpose-built for cruise lines, with only one ship per day (I heard they were going to expand to 2, but I don't know if that's happened since we sailed in 2016). I don't know if the tour operators have additional seats for sold out tours, simply because there's no one else to sell them to other than those coming on the
DCL ship that day, so it may make sense to them to offer all of their slots through Disney. I do know when we visited on another line, there were plenty of people getting off the ship and appearing to book additional tours in the tour center - but I have no idea if availability was over and beyond what was available through the cruise line, or if they were simply selling through the tours that weren't sold out. I also don't know if there's any price differential vs. cruise line pricing.
