DTMF is too low freq and too much difference in the low vs. higest freq tone.
Interesting you should say that. I just did a quick Google on “DTMF underwater” and got this. Are you going to tell NASA or shall I ? 
http://www.fastcompany.com/3015636/scub … -from-1963
BY ALICE TRUONG
Could the key to improving underwater communication be a technology developed for landline phones in the '60s?
At a hackathon earlier this month in San Francisco, oceanographers and scientists from NASA, the California Academy of Sciences, University of California Santa Cruz, and other institutions gathered to figure out how to improve on the current tools of choice for marine researchers: waterproof paper and pencil. The event was hosted by iDive, which is launching an underwater iPad housing this fall. The company is founded by a marine biologist, and the technology originated from a research lab in Saudi Arabia funded by the king.
The winning team of four–all with science and tech backgrounds, including a 15-year-old developer–came up with ScubaTone, an iPad app that relies on old-school technology created by AT&T in 1963 called dual-tone multifrequency signaling, or DTMF for short.
“DTMF was invented 50 years ago for telephone lines before we had rotary disks,” explains Roberto De Almeida, a Brazilian oceanographer and developer at Sunnyvale, Calif.-based Marinexplore and member of ScubaTone. “DTMF is nothing more than the sounds a telephone makes when you press the different buttons, each one sending a signal composed of two different frequencies.”
DTMF isn’t the issue… the problem is in the receiving and sending transducers for water - and the attenuation in water at the lowest vs. highest DTMF frequency. Loud enough, and with the right bandpass filter on the receiving side, one can get some range, as does a beeping sonar. But just a small number of “digits” per second.