Depending on how many are in your party, calling would be the best thing to do.
I would have thought you need to book. Try sending them an email or calling them
They do not require for you to call and make a reservation, however I would recommend it due to the fact they do have a high volume of customers on a regular basis. I highly recommend this excursion.
We made reservations in the morning for noon time when we went with a party of 6
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