帮我翻译一下,谢谢,very few of the fifty pence pieces and pound coins

帮我翻译一下,谢谢,
very few of the fifty pence pieces and pound coins,这里的coins,和 pieces都是硬币的意思么
above the surface of the water,the towers rise to a height of nearly 700 feet,这句话里above the surface of the water怎么解释啊,还有rise to a height 是词组么, the platforms extend to a depth of over 100 feet under the sea,这里的to a depth是词组么,