The submarine must crash dive to avoid the enemy ship.
crash dive
idiomAn emergency maneuver by a submarine in which it suddenly descends, especially to considerable depth, usually in order to escape an attack or avoid a collision.
When the radar detected an incoming missile, the captain ordered the crew to crash dive immediately.
Facing imminent detection by the patrol vessel, the commander initiated a desperate crash dive, plunging the vessel into the silent, crushing depths of the ocean.