From the Celebrity Cruises website:
"Whether on the ship or in a port of call, stylish resort wear is fine for both men and women. For warmer climates, this includes walking shorts and polo shirts, swim suits and cover-ups, lightweight slacks or skirts, and a light sweater. When visiting cooler climates, casual clothing that can be easily layered for warmth or comfort is recommended, along with a raincoat or umbrella, waterproof hat and gloves.
On all Celebrity cruises, it's wise to bring along comfortable, rubber-soled shoes, hat, sunglasses and a good sunscreen. Remember to pack any prescription medications or other personal care items you'll need"
On cruises of up to 6 nights (which, as far as I can see, doesn't include any of the Galapagos cruises), 1 night is 'formal', with the remaining being 'smart casual & above'. [See below for exception].
For cruises of 7 to 11 nights, the are 2 'formal' evenings. [See below for exception].
Above that, there are 3 'formal evenings'. [See below for exception].
(NB: The preceding three paragraphs do NOT apply if you're on the Celebrity Xpedition, as a casual dress code applies at all times - See below
The website suggests the following as examples of 'Smart casual & above':
"Ladies: Skirt or trousers (no holes, rips or tears) complemented by sweater or blouse.
Gentlemen: Trousers (no holes, rips or tears) with sports shirt or sweater. Shirts must have sleeves"
The suggestions for 'formal' are:
"Ladies: Cocktail dress, gown or dressy trouser suit
Gentlemen: Tuxedo, suit or dinner jacket with trousers"
The following note is added for further guidance:
"T-shirts, swimsuits, robes, bare feet, tank tops, baseball caps and pool wear are not allowed in the main restaurant or specialty restaurant at any time. Shorts and flip flops are not allowed in the evening hours. As a courtesy to our guests, we respectfully require and enforce compliance with the dress code at the entrance of your restaurant of choice. We thank you, in advance, for your cooperation and understanding. Additionally, guests are asked to follow the "Smart Casual and Above" dress code in the Celebrity Theater for all evening performances.
The daily program, delivered to your stateroom and available at the Guest Relations Desk, will be your guide to the appropriate attire each evening. A casual dress code applies at all times onboard Celebrity Xpedition"
(As it appears that all of the Galapagos cruises are on the Celebrity Xpedition, you can probably ignore the 'Formal' references above but, just in case you're on another ship, I've included them anyway).
Source:
http://www.celebritycruises.co.uk/frequently-asked-questions/