white horse

Roald Dahlの短編 "Dip in the Pool" からの引用です。
'The sun deck, please,' he said to the elevator man.
The wind caught him full in the face as he stepped out on to the open deck. He staggered and grabbed hold of the rail and held on tight with both hands, and he stood there looking out over the darkening sea where the great waves were welling up high and white horses were riding against the wind with plumes of spray behind them as they went.
上の文中に出て来た "white horses" ですが、文脈から白波を連想させます。辞書で確認します。
・Vocabulary.com: a wave that is blown by the wind so its crest is broken and appears white

・Collins Dictionary: a wave with a white broken crest

・Wiktionary: A wave that is blown by the wind such that its crest is broken up and appears white and foamy; a whitecap.