While using DTMF tones may work on an individual basis (I really don't know) wouldn't it result in total chaos in a situation like the Sub Regatta with several boats in the water at the same time? In other words, how would a boat "know" which tone was meant for it?
On using LF for model subs, the resonant wavelength for antennas at those frequencies is measured in miles not inches. The efficiency, and thus energy transfer, of any antenna that could be used in a model sub would be abysmal, on the order of probably less than 0.0001%
"By doing just a little every day you can gradually let the task completely overcome you." - Wil Baden