There are many and they work on several principles. Conduction, capacitance, refraction, etc. They are even a hardware store item. Google moisture sensor.
Do you want to know if something is immersed, or if something has been immersed but is not now? What is the "body" that you want to know if it is immersed?
You should search on "Point Level sensors". Point Level sensors are commonly used as high and low level sensors to help in the overfill or underfill of vessels and tanks for example. There are various technologies available as stated in the earlier reply from another member. I do manager these particular products for Siemens and you can contact me direct at billy.jenkins [at] siemens.com for more information if you wish.
By body do you mean a live person, a live person who will no longer be with us as a result of the immersion, or just a plain old fashioned corpse? If alive, I suggest asking them to flip a simple switch once they're immersed. Use low voltage if you want to avoid electrocuting them.
If the person is alive but expendable, that would require another technology. In that case I suggest using a "dead-man" switch, which works on the assumption that the person is alive and holding the switch contacts in the "not immersed" or "not dead yet" position. Once they're fully immersed and have expired, they'll release their grip signalling that they are fully immersed and in need of replacement. A simple SPST push button switch should do the trick. Again, I'd use low voltage if you prefer the "loss of grip" to be the result of drowning rather than electrocution.
Now if they're dead to begin with, I don't know. Perhaps someone else could help with a solution for that one.....
I hope this is at least as helpful as your problem description....
The thing I need specifically is a detector or a sensor, which to be mounted on a swimer's hand, to detect accurately when the hand is under water and when is above water surface.
This application belongs to a certain program in researches of Enhancement Training Programs of Blind athelete people.
Hmmm. Any simple sensor will make a circuit based on the residual water on the skin of the swimmer. I would call Don Koeneman personally at Ametek Drexelbrook and ask him for a solution. This is a non-commercial suggestion.
Don has expressed his significant dislike for me personally many times over the last ten or so years, but he does know what he is talking about, so I strongly recommend you contact him...Ametek might be willing to work with you since it is a project to assist disabled persons.
That sounds like a good fit for optical diffraction. It could be completely enclosed and sealed. The elements are angled such that in air they do not meet but in water they do. Or vice versa.
