Because 2.4ghz only penetrates a few inches of water, not so good for a sub. 75Mhz,50Mhz, or 27mhz for hobby rc subs.
Here she is running trials in a real pond. The static dive test worked both times, I don't remember the wide turn radius in my last one but for its first time out I am pleased. I did not fight it at this this event because I have the rudder on the wrong stick and it will be a major effort correct.
I got a video of the sub diving, and I put it on youtube as unlisted, I could link it here if people are interested.