last minute snorkeling trip

We will be visiting St. Lucia for the second time in November. Last time we booked an excursion a few weeks beforehand with Spencer Ambrose. We had a great time and highly recommend him. This time around we would like to wait until we arrive at port to schedule a snorkeling excursion in case the weather is not good. Have any of you booked last minute like that from the port in St. Lucia? Do you know if independent tour operators similar to Spencer Ambrose will take extra guests? We can't imagine that it would be a problem but don't want to just assume either.