A machine and method for anticipatory sensing and intervention to avoid swimmer entrapment, entanglement or evisceration; with a proactive, pre-entrapment, ultrasonic sensor assessing the relative hazard of swimmer proximity to a drain cover. A transducer launches waves into the suction piping and/or drain system, and receives echoes from the drain cover, swimmer limbs, hair or body, and water or wall surface parallel to the drain cover. A transmitter electrically energizes the transducer launching waves into the suction piping and/or drain system. A receiver/processor detects the echoes analog signals from the drain cover and water beyond the pool drain. A Logic and Control element converts the detected signals into reliable information regarding safety/hazard status for a swimmer near a drain. Predetermined logic provides automatic pump shutdown, and alarms as required; including a missing drain cover. A pool alarm mode detects that an object, such as a small child, has fallen in.