This is one of those movies, like Lost in Translation or Eternal Sunshine, that you are either going to love or hate. I don't think it will allow a middle ground. As such it's maybe one to rent before you buy.
It's also one of those films that are about nothing and everything all at the same time. So, at once, it's a fable about friendship, the difficulties (and rewards) of marriage, the insidious nature of professional jealousy, the pain and joy of fatherhood. But it's also about pursuing a dream in the face of ridicule. It's about camaraderie, bravery and foolhardiness.
But, arguably, it's also a critique on the sort of men who never grow up and who, metaphorically, will forever yearn to be the omniscient captain of a boat, venturing to sea to hide from their responsibilities and commitments.
And, then again, it's about the hunt for a great big fish.
Strange and beautiful. Frustrating and mystifying. Elegant and elegiac. Surreal and realistic. All of these things and none of them.