Wee bit out of my comfort zone with this one but I've seen a few documentaries and been on a few aquarium tours. Hopefully someone more qualified will verify or correct me:

First, their eyes are located high on their head, which means they can keep their eyes above the water surface when swimming. This allows them to see what's going on around them without having to open their eyes underwater.

Second, they have a third eyelid called a nictitating membrane, which is a thin, translucent eyelid that closes from the inside corner of the eye, covering and protecting the eye while still allowing the animal to see. This membrane also helps to keep their eyes moist underwater.

Third, they have a clear membrane called the cornea that covers the eye, protecting it from debris and sand. Additionally, their eyes are covered with a layer of transparent skin, which acts as a shield against any foreign objects.

So, even if sand or debris manages to get past the eyelids, the clear cornea and the transparent skin provide an extra layer of protection, preventing any irritation to the eye.