Do keep checking. Some people will cancel shortly before they have to commit (three days prior to sailing), which is also the deadline to sign up online.
Also, if you are trying to get a bunch of tickets for that excursion, don't put in the full number you want. Put in a lower number, maybe even just one. If you ask for 6 and their are 5 left, the system will just say "unavailable," so doing it that way, 6 would have to open up at the same time for you to succeed. Take the 1. Then ask for 1 more, etc, until they are truly out, then hopefully, more will come available in a few days, and if towards then end, you can't get all the tickets you want, and don't want only part of your group to go, you can always give up your tickets and make someone else's day.
Good luck!