Marc, don't worry about being waitlisted for an excursion, the waitlist always clears, usually within a short time after you do your booking. I think what happens is that they start out with one or two buses and then add as demand requires. I've never heard of anyone being denied a tour. I had the same question about Sandakan, but wonder if they're trying to "limit" some of the tours there because the infrastructure probably isn't that great. We signed up for one of the free tours there and we're waitlisted. If we decide we want to do one of the paid tours, we'll sign up onboard so we can use onboard credit. Again, have never been denied a paid tour--even at the last minute.