Heart_and_Soulunderground currents
Also, I should mention is that the other danger is not knowing which way is up when underwater. You can have a bad landing, end up underwater and dis-oriented (which might've been dark due to the muddy environment and then end up not swimming correctly to the surface (as coupled with those currents.)) Eventually you can end up panicking.
Typically you'd either swim toward the light, watch your bubbles to discover up, or pull yourself up with a surfboard strap (which he wouldn't have had.)
Anyhow, not really the thread to discuss this, but important to mention.