A very good question, whilst doing a bit of research I found this very good website, and although it does go into a lot of technical detail... I think I understand the idea of it.http://www.qsl.net/vk5br/UwaterComms.htm
Basicly it is saying it isn't the best idea (as most of us know) but it does say it is possible. If in sea water then it is saying you don't have a chance because of electrical conductivity and the such unless you have a huge amount of power, a long antenna and you use a stupidly low frequency.
In fresh water however it states 'Fresh water lakes and rivers have much lower electrical conductivity than the sea and underwater transmission distances (or depths) up to 30 metres appear feasible using the lowest frequency amateur band of 1.8 MHz. Even larger distances (or depths) could be achieved if a lower frequency band allocation is available. *(Attenuation at 180 kHz is reduced to around 18 dB per metre in seawater and around 2.2 dB per metre for fresh water.)'
So basicly it looks like it is possible and you shouldn't have much problem over a short distance but considering bluetooth uses 84Mhz (I think) then you may have issues as this is considerably higher than the 1.8MHz which they used.
In tests that other people have done, they got it to about 10 inches using mobile phones and the such.
Another thing I think may cause problems however is if you have a heavily lined walls that are made out of metal as these could reflect your signal and cause other problems.
Hope this helps and good luck!