Hi. We will be creating a smartphone controlled underwater drone for our robotics class. Can I use a Bluetooth radio to send signals and connect to my smartphone? The device would be at least 10 feet below the surface. If not, What would be the better options? Thank you 
Probably not.... since those kinds of signals get weak very quickly in water mediums.
That's why they use acoustic transmitter and receiving devices..... ultrasonic transducers, hydrophones etc. These usually work at frequencies a little bit above audible hearing range.
will it still be possible for us to connect and control our device through a smartphone? or should we create a controller device for it? thanks
Why not a little bouye than ascends to the surface? With a fishing line in a tensioned spindle...
Maybe too much drag...
High frequency (HF/VHF etc.) radio waves don't travel very far in fresh or salt water. Bluetooth is transmits at ~2.4GHz so you'll have a range of only a few 100's of cm in water. As travis_farmer mentioned the military use "very low frequency" (VLF) for communicated with submarines but these transmitters are big and have lots of RF power behind them.
You'll be better off looking at a tethered / umbilical cable solution. Google ROV's and projects such as OpenROV.
You should be able to find some examples of wargamer type people using FM frequencies for their submarines. Old school ~27MHz radios. But that still won't get you much distance; just a meter or so underwater.
Qdeathstar:
Why not a little bouye than ascends to the surface? With a fishing line in a tensioned spindle...Maybe too much drag...
We actually considers making a bouye that will connect to the device (which will go underwater) with a tether. Then the bouye will be the one who'll connect to the smartphone via Bluetooth. Do you think it will work?
Yes, that should work.
Check the openrov project as they have done something similar
travis_farmer:
what about a low power VLF system? I read somewhere that US Military subs use a VLF array to send and receive data transmittions.I don't know much more than that, but I thought I would pass along the idea.
~Travis
It's fine, if you've got a few linear tens of kilometres for the antenna.
The RF sender on my first-stage regulator (I think around 10-20kHz) barely manages the length of my arm to my dive computer.
How about a green laser?