Last time to book excursions?

[zeldacruiser]

What's the last day you can book shore excursions before the cruise? I know you can book onboard, but I seem to recall a cutoff date of maybe a few days beforehand for advance booking. Only six days until our cruise and I haven't booked anything yet, so maybe it's too late for advance booking? I realize many things will be booked up, but we weren't planning on anything really exciting - maybe snorkeling.