Nictitating membranes

Nictitating membranes

I have no idea why, but in addition to all of the previous, I have also developed an interest in the nictitating membranes in birds and mammals.  These are the so-called third eyelid that crosses the eye beneath the usual eyelids.  Humans don't have them, but cats and many other mammals do, and, I think, all birds.  As far as I can make out from my own observations, all birds that hunt or forage with their heads underwater (well marine birds, at least) have transparent nictitating membranes - e.g. gannets, shags, gulls, oystercatchers, auks.  Land birds have opaque ones.  So, are the transparent nictitating membranes used as goggles to protect the eyes from damage from seawater or microorganisms and parasites in seawater?  And might they also act as an additional lens?  I'm told that dippers have opaque nictitating membranes, which could mean that freshwater does not present the same dangers as seawater.  I wonder if there is therefore a difference between marine and freshwater ducks?