Riddle (Ocean)

Always breaking, never broken
Always still but full of motion
In a sea-shell find my token
A metaphor of emotion.
I circle earth, broad and deep
Turn and run, but have no feet
Full of tears, yet cannot weep
Rest on beds, though never sleep.
Ridden by mares
Fathomed and unfathomable
I carry men upon my back
Can swallow whole a humpback
In the sun I turn to diamonds
In the rain I don't get wet
Collector of pearls, full of monsters
Jewellery-maker from broken glass.
Watch me come
Watch me go
Watch me wave out
To your toe
The greatest and least
Know me best
A natural wonder
I cannot be divided
Summer and winter
People fight over me
Midnight and noon
At the same time
Morning and evening
There's really only one of me
Waiting for no-one
A good old salt
I blush at sunsets
Connect continents
Unite the world.
What am I?