The RF module sounds like a good idea. Since both the person and the object with thrusters will be at the surface almost 100% of the time I don't think the water's low frequency requirement will be too much of a burden. The reason a chord is not an option is because it would simply get in the way. Now does an RF module transceiver have 2 parts, one part that you would wear(in my case) so you could remotely control the thrusters, and another that's fixed on the object which recieves all of the data from the remote part? Also getting back to the fail safe, how would you go about programming these kinds of things, I'm relatively new to this kind of technology and I'm not sure how I can really set up the remote control effectively. Do you have any tips on how I can figure this out?
You may be able to use something like this:
http://www.ebay.com/itm/DC-12v-10A-relay-1CH-wireless-RF-Remote-Control-Switch-Transmitter-Receiver-/221283333405?hash=item338583b51dSimilar devices widely available.
If that doesn't work for you, you have Bluetooth options. Bluetooth is a relatively short range technology commonly used for headsets for phones, keyboards, etc.. There are a wide variety of other uses so there are a lot of different devices out there.
Here is a bluetooth remote relay that might be suitable for switching off the thrusters:
http://www.ebay.com/itm/Lazy-Bone-Android-Bluetooth-Smartphone-cell-phone-wireless-relay-switch-/230939710949That comes with code to use with a cellphone.
Whether you can find a transmitter / receiver combination ready to use, I don't know. You'll need to look around. It's possible.
All this can be assembled, coded and built with commonly available parts and a microcontroller like an Arduino. Parts for both ends would be around $50 and perhaps less depending on how sourced. You would have to write some code, but there is a good bit of starter code out there. If you decide to go this route, the Arduino's have a huge support base and you can get guidance at the Arduino forum:
http://forum.arduino.cc/