i know hydrophones are used for this technique, but after seeing this: http://audiocookbook.org/sound_design/sounds-from-a-contact-mic-frozen-in-water-ice/ i thought, is it safe for contact mic to be in water? why do people use hydrophones then and not contact mics for underwater recordings?
The piezos used in contact microphones pick physical vibrations from solid surfaces as you can see here, for example: http://vimeo.com/31926631 In that case of ice recording it makes sense to use those, but I don't think (and I hope someone correct me if I am wrong) a contact microphone under water -- in any other state than solid -- won't pick up much vibrations which the piezo is able to transduce to an electric signal. For my experience the contact mic needs to be well attached to the surface you are trying to use, and sometimes you even have to exert some pressure in it with a tool or your own hands.
For the underwater recordings, I know it works acceptably well some simple small omnidirectional microphones with the advices given above.
As far as I'm aware, some hydrophones are actually made with contact mics (or piezo transducers) sealed inside a protective housing - so I think your logic is sound.
Actually, Jez Riley French makes some and it seems from his wording that that's how he makes his: http://hydrophones.blogspot.co.uk/2009/04/hydrophones-by-jrf.html
You also could use a SM58 or any another microphone, put a condom over it, make a lot of gaffertape around it until it's dense and hold it in the water. (I would suggest to take your cheapest microphone :-). A hydrophone is a specialized microphone for the use in the water. For example, if you drive thousands of miles to the north pole and want to record something special under water, you look quite bad if your SM58 in the condom get wet and breaks of the salt water. You look much worse if your expansive Neumann breaks and the most bad thing is, you can't make records anymore and you don't have a music shop around the corner. You can forget your records or drive back an get another one.
So if you have a hydrophone, you make your underwater record and get great results - hopefully. It's a kind of safety and it probably makes a clearer record. It's expansive but for important records I would really consider about it.
It always depends on the situation. ...and I'd love to have one.