In addition to any tourist visa for Ecuador and meeting the onward travel requirements for your visit to the Galapagos Islands, you'll need a specific permit for your visit as the vast majority of the archipelago is considered a national park.
At the time of writing, it's two separate fees of US$100 and US$20, totaling US$120.
When you're traveling with Celebrity as part of a longer cruise tour, including a Celebrity pre/post-cruise of Quito or Machu Picchu, you'll be delivered onto the islands on a charter from Quito and Celebrity's expert service will see to it that your permits are arranged on arrival - it's included in your cruise tour.
But if you've only arranged the cruise with Celebrity and plan to enjoy a custom pre- or post-cruise experience, what happens then?
Celebrity's Galapagos team has informed me that guests arriving on their own should pay the fees and receive your permit as an ordinary visitor to the islands, but keep all your documentation and receipts for reimbursement after your embarkation.
Their team will take care of the rest once you're on board and ready to meet some wildlife!