Submariners using 2.4ghz are generally enabling it by extending the receiver antenna using coaxial shielded cable to run it up a scope or some such element. Some have managed to get a couple of inches of depth in fresh water using long range 2.4ghz systems, but results seem a bit variable, so generally you should consider 2.4ghz only works dependably if you run the antenna above the surface.
A few modellers are now using 866/900mhz LRS systems, which can give a few feet depth in fresh water, which for many is enough. These also give the advantages of frequency hopping, more channels, DSP with software encoding and smaller aerials than the older low frequency systems.
Some other modellers, mainly on the AMS site linked by Fred have gone for a more DIY approach with LORA 433/459mhz boards and Arduino boards, which may suit.
Range seems decent but the latency looks high on the system presented, fine on a slow model, but may prove bit too sluggish for faster craft.
Regardless such a system will require an aptitude with software and electronics which is generally sufficient to scare off the majority who will favour a ready built solution.
