Water immersion sensor
One of the simplest ways to detect this would be to use a floating switch similar to the ones used in sump pumps in the US. Basically it consists of a float that has a density higher than air but lower than water that can move up and down a short post. When in air, the float rests at the bottom of the shaft, when in water - at the top. Placing contacts either at the bottom or the top such that the float would close them would allow you to electronically determine if the object is submerged or not. 

The problem with Bluetooth is that the antenna would detune when your object is wet but not submerged making the problem more complex.