I love the cleverness of construction of this poem, Jim.
The single line format makes the poem itself into a picture of the scene for us – like a boat on the horizon. The middle of the poem is the word "rocks". It stands like a fulcrum, or the middle of a seesaw, to balance the two halves of this haiku. But ironically in this particular work the word itself is ambiguous – is it the boat that rocks, or are there physical rocks? And so on that word, the interpretation of the poem itself "rocks", it moves and shifts up and down.