> "Bent to his breast" ... seems to me an obvious ellipsis for "bent down as part of the process of lying on his breast in a swimming position in the water"

Or an equally obvious case of adding a word (and a rhythmic syllable) that doesn't alter the meaning of the shorter phrase. A form of pleonasm, if you will.

I submit, however, that Richie's original question ("What does it mean?") has been answered.