For just 2 m range, how about a low frequency magnetic field, not radio waves*?
Have a coil in each device, and drive a current into it at (say) 22 kHz for 0 and 44 kHz for 1.
Receiving would use the same coil, or one with more turns, connected to an audio amplifier, automatic gain control, and some sort of detector. Even straight into your micro, depending on what it has.
The self-resonance of each coil should be high, at least 100 kHz, and the inductance low enough that you can get reasonable current into it without unreasonable voltage. There's no gain from resonance, you don't want a high Q coil here at all.
This principle is used by heart rate monitors, which work (badly) under water. And I can relate some childhood experiments with a coil around the room, connected to an audio amplifier in place of the loudspeaker, and a search coil picking up the music anywhere nearby.
Fresh water will have almost no effect on your magnetic fields. Seawater, I don't know.
Its quite easy to experiment, with a scope and a signal generator or oscillator.
.* In fact all changing magnetic fields create a changing electric field, and hence an electromagnetic wave, but here we are in the near field where the magnetic field of a loop is dominant.