40mHz will go deeper than that, in clear fresh water I have had my subs down 5 metres but wouldn't recommend it though. The higher the frequency of radio waves, the less likely they are to penetrate and in fact, the old 27mHz gear had better penetration as the frequency was lower. Salt water will actually block radio waves quite effectively, especially at these high frequencies and thats why real subs have ULF (ultra low frequency) aerials or 'towed arrays' which are streamed out of the aft end or out of pods placed out of reach of the props. These are effectively a very long wire which is held on a spool and trailed behind the boat and they are of this length as they are tuned to transmit the exceedingly low frequencies they operate under and are capable of transmitting right across the globe.
So, high frequncy transmitters have short aerials, ie note the tiny aerials as fitted to 2.4gHz radio gear and the longer ones as fitted to lower frequency 40mHz equipment. This is why our subs won't work with the new 2.4 gHz radio.. even in fresh water.