They waited for low tide so that the sea level would drop about two meters and then sailed under the bridge.
「海面が2メートルほど下がる干潮時を狙って橋をくぐり抜けた」は、
They waited for low tide so that the sea level would drop about two meters and then sailed under the bridge.
※low tide 干潮
※so that ~ ~するために
※sea level 海面
※drop 下がる