Below is a video of my friend William Trubridge completing a dive to 393 ft. on one breath of air.  He did it with a monofin and constant weight, meaning the weight belt he uses to go down he must bring it back up as well.  William is one of the best in the world holding most of the freediving records.  This is an excellent video for those who want to understand competitive freediving better.  There are some great explanations used throughout the short clip.