free as the wind
First, in the old days of sailing at least, ships were driven by the wind. No noisy engines, no pollution; just like the catamaran in this picture, you are as free as a bird to be carried on the wind.
Next, once you are far out to sea, or in blue water as sailors call it, there are no boarders or boundaries. You are doubly free – sailing an ocean that nobody owns – and carried by a wind that is free.
I suspect, therefore, that my longing for the sea and my father’s too is not just about the fun of sailing a small craft like the one in this photo or a more sturdy blue water capable yacht, but a longing for a borderless world where people can go wherever they will.