RHYMING RIDDLE

I come and I go, more than once in a day,
And yet I've always been here - for eons my stay;
A bearer of life, yet I bring fear and dread,
A destroyer of life, yet millions by me are fed.
Artists and poets my presence inspires,
My moods are varied, my journey ne'er tires.
My breadth was once broader, but my time comes again,
Mankind will decide if I'm a foe or a friend.

Answer - in verse:

The Sea is my name, or Ocean some say,
My tides wax and wane, twice each short-lived day.
Many a life my waters have seized,
But I give back aplenty with food for your need.
My reach was once broader, eons ago,
Melting ice insures that again I will grow.