Well it seems that everytime you fall into the water you get around 14 seconds before you start running out of air. So maybe the bots could have some sort of internal counter that counts down 14 seconds and we'll say at 11 seconds they go back up for air to the nearest node and wait for 3 or 4 seconds then go back under if needed. Could that be possible? From the looks of it it seems you got it to where they know there underwater, i figured that'd be the hard part.
It's just an idea in case you were interested.
Edit: Also mal, what's the Node Connections thing for? I seen it on the in-game node HUD.