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.