Magnetic modulation/ induction
Is there a case for radio over magnetism underwater?

You can send and receive data using radio underwater but you will find that sea water and fresh water are quite different mediums: -



The graph is comparing seawater with Adelaide fresh water and yes, the graph is a bit poor quality but the formulas are here: -


Attenuation (α) in dB/metre = 0. 0173 √(fσ)
where f = frequency in hertz and
σ = conductivity in mhos/metre (siemens per metre)


The graph tells you attenuation per metre and is taken from THIS document. So why persist with radio waves when you can do it magnetically? 

Answer - In the short range "arena" magnetism wins but as distance increases the voltage induced in a receive coil falls as distance cubes (as does the E field when not conjoined to a H field like it is in a proper EM wave). If you transmit a radio wave then this is a proper EM field and individually E and H falls as plain ordinary distance. That's the beauty of radio - you appear to get something that is totally better than the components that it's built from.

See my answer to this unrelated question for some more detail and also this answer which has the following pretty picture: -



The picture above shows the formula for flux density at a distance from a transmit coil and note that as Z gets dominant over the coil radius the denominator becomes \$2Z^3\$ i.e. an inverse cube law.

So, my recommendation is to strongly consider a proper radio wave or at least compare the levels of signals you are likely to receive versus distance. Also, read the very excellent document with the bold link under the bullet points. You can learn a lot from it.