Normal vision for the fish is for light going from water to the eye. This condition is met if the fish wears goggles filled with water. Cover half the lens and you cut out half the illumination of the light. But you don't cut out half the image, as is commonly and mistakenly thought. If light had the same average speed in glass lenses that it has in air, no refraction of light would occur in lenses, and no magnification would occur.
Magnification depends on refraction, which in turn depends on speed changes.
A magnifying glass will magnify less under water because there is less change in speed as light goes from water into glass. With less speed change, less refraction and therefore less magnification occur. A pinhole camera with two holes simply produces two images. If the holes are close together, the images overlap. Multiple holes produce multiple images.