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.
Furthermore, the waves being 'gusty' indicates a liveliness and a joy for living. This is a privilege lacking in these poor children's lives. Their circumstances are so dull and drab and lacking substance that that they appear lifeless and/or ill. They have no expectations and are drained of life. Spender refers to their 'pallor' which suggests that they are extremely pale and lack the vivaciousness other children in grander circumstances would have.
Also, on a more literal level, these children's families lack the resources to ever take them to the ocean where they would be able to enjoy the wonders of nature. They are denied the opportunity to ever see waves breaking on the shore or smell the fresh wind that drives these waves. The prospects for them are grim. The 'gusty waves' denote a life of privilege and opportunity which is sadly lacking in these poor slum-dwellers' lives.
The poem is a damning critique of the neglect suffered by poor communities and the lack of opportunity they have to endure. They suffer lives of drudgery and pain, lives without any hope for betterment. Stephen Spender brilliantly expresses his disdain for an indulgent society which feeds off the labour of the poor, but treats them with disdain whilst their lives continue in splendour and privilege.