When a glacier is actively calving, it can put down so much ice that the fjord is hazardous to navigate. In our case last week we went to an alternate glacier (Dawes versus Sawyer) because Tracy Arm was blocked up pretty nicely. I think the captain said if we went there the closest we'd get would be miles away from the glacier.
The "glacier day" aboard ship is typically an at-sea day so you can't leave the ship in a quest for an excursion. But some of the ports do offer excursions to nearby glaciers.
Dave (and others) Endicott Arm (and Dawes Glacier) is the other half of Holkam Bay, and in very close proximity to Tracy Arm. That's an easy substitution.
Hubbard Glacier is a bit of a different story. For one, there's not much else nearby, so a quick substitution if it's blocked is unlikely without several days of re-planning, port re-scheduling, etc.
Also, someone asked why it was blocked. Hubbard is advancing so rapidly (again) that one side of the glacier has almost completely blocked access to the bay. This isn't just heavy calving activity where the icebergs will clear with the next tide, it's an actual solid wall of ice and dirt pushing its way across the entrance to the bay. If the entrance is mostly blocked, the large cruise ships can't enter, and the most you'll get to see is a small corner of the glacier from several miles off. The real fear, however, is that if the bay is completely blocked, it will cause the sea water (salt) to spill over into the nearby Situk River (fresh) which would be catastrophic to the river, salmon, and life that depends on a clean river.
