A full underwater housing for a mirrorless or DSLR body is expensive, bulky to travel with, and honestly overkill for most travelers. The good news: the Galápagos is one of the few places on Earth where an action camera and a snorkel mask get you shockingly good results, because the marine life simply doesn't swim away from you.
The minimum kit that works
An action camera with a flat lens (not the stock wide dome, which distorts underwater) or a dedicated dome float attachment, shot in the highest available resolution and frame rate. A wrist strap or float handle is non-negotiable — losing a camera to the current happens fast and often. If you want a step up in quality without the cost of a full housing, a compact camera in a basic waterproof case with manual white balance control goes a long way.
Working with what you've got
Get as close as you comfortably and respectfully can — water absorbs color and contrast fast, so distance kills image quality more than almost any other factor. Shoot slightly upward toward the surface when possible; it backlights the subject against the brighter water above and produces a much stronger silhouette-to-detail range than shooting downward into the dark.
Where to aim your time
Bartolomé and the waters off Isabela are reliable spots for Galápagos penguins hunting in the shallows, especially August–October when the cold Humboldt current peaks. Green sea turtles are common almost everywhere there's seagrass, feeding calmly and unbothered by snorkelers who keep a respectful distance.
Final thought
You don't need a $2,000 housing setup to bring home a genuine underwater portfolio here — you need a camera that survives the water, the patience to let the animal come to you, and enough sessions in the water that at least one produces the shot.