How do I get started?
 

I have airplanes and helicopters. Would the same 2.4 ghz radio work for a sub?
I have a chocolate lab who loves to dive, and would like to give her something to chase/ fetch. Nothing fast or fancy, just something she can play with.

2..4GHz will not work underwater. You would need a lower frequency, like 75MHz in the USA or 40MHz in United Kingdom. There is a group working on 433MHz. It shows a lot of promise for not only navigation, but for sending back data (such as depth, temperature, rpm of motor, and compass direction).

Will a 72 mhz work?

It will, but is an illegal frequency to use for land or watercraft.