Your only options for a submarine is to use one of the European ground frequencies -40MHz and the older 27 MHz bands. No need to retune a 75MHz(which propably isn't possible anyway) as there seems to be a far wider choice of 40Mhz set available. 2.4GHz gear does not work underwater.
Hitec Zebra 4 (40Mhz) at around £40 is a good basic 4ch set. If you can strectch a bit more, one with a few more channels and accompanying computer - say a 6ch Futaba or Hitec- will be more useful if you ever plan on building other more complicated models.