The metaphor, 'gusty waves' has a number of interpretations. Firstly, 'gusty' refers to a strong breeze or wind and the implication is that a powerful breeze has lifted the waves and drove them on.
Spender's repetition of 'far' emphasises how far removed these poor, ailing children are from a normal life. Their bleak and impoverished conditions imprison them and gives them no hope for the future. There is nothing in their lives that can drive these children towards a better future, just as the winds drive forth the waves and animate them. These children are not animated or excited about their lives - there is no ambition, just a profound apathy and acceptance of their pathetic existence. They are completely removed from the desire to attain a better life and are overwhelmed.