Question

What is being compared?

He sprang through the waves and caught her, then crouched amid ships,
Avoiding sure death, while a great wave bobbled the boat
This way and that with the current. As when in autumn
The North Wind blows close-clinging ball of thistles
About the fields, even so the winds were driving
That boat of his this way and that on the sea.

Answer 1

while a great wave bobbled the boat that way with it current as when in attum the north wind blows close clinging ball of thristles

Answer 2

Answer is: The autumn north wind blowing thistles in the field is being compared to The current in the waves blowing his boat about in the sea.