請(qǐng)先看across與through的區(qū)別
through表示“貫通、直穿、透過(guò)、穿過(guò)”的意思,即是從一頭（邊）貫穿到另一頭（邊）.
例The river runs through our city.這條河流經(jīng)我們市.
He passed through the hall.他穿過(guò)大廳
across表示“橫穿、橫過(guò)、橫渡、橫跨”
例：I swam across the Changjiang River 20 years ago.
20年前我橫渡了長(zhǎng)江.
Look left and right before you go across the street.
過(guò)馬路時(shí)要左右看.
所以用swim across the sea