Hi
Are you building an AUV "Autonomous underwater vehicle" ??
Blue tooth is useless underwater.
Radio waves have some transmission BUT are very limited on their distance underwater. Fresh water Approx 6 Meters .Sea Water 2 Meters.
What is the Max depth you require ??
I have used a Gopro camera taking photos with time lapse..Worked well for me.
If its a RoV then Nick has done some good work with RS458 http://www.gammon.com.au/forum/?id=11428
Also Bill uses a really easy library EasyTransfer Arduino Library « The Mind of Bill Porter
You can get high pressure glands that will cope with extreme depths. But the cable will also be special.
Regards Antony