Buy your ticket(s) online at the lagoon website, then make a reservation (online) with 7 days of your visit. Reserve as soon as possible or you may not be able to get the day you want. If going in June, look for the promo codes to get a discount for students (k-12) and teachers.
You have to purchase a ticket first then there's a link at the top of their website to make a reservation - they can be made up to 7 days in advance
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