designs may be both functional and have a certain aesthetic appeal as discussed above the portable reception units would receive alarms 2824 from annunciation units 2812 after said alarms were received from tags 2802 worn by swimmers communications may be by bluetooth protocol a completely portable embodiment is suitable for families visiting a body of water it can consist of tags one or more portable battery powered smu units a battery powered supervisory control unit and or one or more portable reception units the supervisory control unit may be configured like a briefcase or u201c boom box u201d a 25 meter long by 6 meter wide indoor pool was equipped with a swim monitor unit the pool had a shallow end 1 meter in depth and a deep end 3 meters in depth the swim monitor unit was mounted at the middle of the wall of the deep end the hydrophone rested on the bottom of the pool at a depth of 3 meters the annunciation unit rested on the edge of the wall of the pool and communicated with a supervisory control unit by radio transmission the supervisory control unit was 3 meters from the annunciation unit a test swimmer entered the water at the midpoint of the pool and submersed a tag in the water the tag was programmed to emit an ultrasonic yellow alert signal at 30 seconds and an ultrasonic red alarm signal at 45 seconds after the tag had been submersed for 30 seconds the supervisory control unit sounded a yellow alert the test swimmer then removed the.