I haven't tried, but it might be worth it to just call everyday and ask about making a reservation. Cancellations do happen. That combined with showing up at the dock - you may just score a cruise!
That might not cut it..........only because if there is a cancellation, they probably tell the Captain not to come to work that day........You don't generally see any Captains hanging around without a "party" to take out.............
you might find an open boat but no one to drive it..........