It seems to me that if they can't move it by any normal means, they could just start pushing cargo containers overboard and fish them out until it's light enough to move.
I could also a lot of plausible reasons why they wouldn't do that:
- it wouldn't work (no equipment to move containers)
- the ship's owner doesn't want to do that and no one has the authority to force them
- the ship's owner wants to do that, but the owner of the cargo doesn't want them to
- the cost of lost cargo is more than the cost of delaying other ships
- it would take too long or be too messy to clean up