From what I've read about Grand Cayman on other websites, I believe they expect to begin welcoming SOME visitors in early November. As the island recovers more, then they will be able to accomodate more.
Also, since many of the excursions on Grand Cayman involve beach or snorkeling or scuba, those shouldn't be affected as long as the operators boats are OK. I think I read a report maybe on cruisecritic.com that said that the sting rays were fine, the turtle farm had some damage but not catastrophic and that the snorkeling and scuba might even be better since the hurricane revealed some new formations when it shifted the sand around.
This is just my memory so it may be faulty. For a fact, DD says that
Castaway Cay is greening up again so I expect that is true of Grand Cayman also. I think
DCL is waiting till closer to each sail date because lots of progress can be made in two weeks so they're making the decision on a cruise by cruise basis.
I know many would rather go to Costa Maya but the folks that booked a year ago were planning on Grand Cayman so I'm sure DCL will get back to the originally published schedule as soon as it' feasable.