I will be corrected if I am wrong, but I believe there are arrival and departure times due to the fact there are a limited number of ships that can come and go in these places so its sort of crowd control. Believe me,,scenic cruising in Glacier Bay is really cool.
You will have days that are just sea days and those are called scenic cruising,,,and it really is scenic. These days are cheaper too because you won't be docked and spending your life's fortunes on excursions and gifts!
The beauty of an Alaska cruise is the "scenic cruising" Whether it is ttime spent in Glacier Bay, , College Fjord or anywhere along the inside passage. Glaciers, floating ice, mountains, rivers, waterfalls, whales, seals, se lions, eagles. This is what Alaska cruising is all about. The time in and out give you an idea of how long you will have to do your viewing.