Hydrophones as contact mics

["davimon100" davimon100]

Am I right in thinking that hydrophones are waterproof contact microphones?=
  If so, it should be possible to hook a contact mic up to pick up surface =
vibrations on an object outside of water, right?
I tried attaching a Dolphin Ear hydrophone to a vibrating surface and was u=
nderwhelmed by the result.  I'm sure the mic is fine but, rather, my techni=
que is flawed: I believe I may not be making a good enough contact - I wond=
er if the air between the hydrophone and the solid is muffling the sound.  =
Can any of you share tips on getting an optimal contact between a hydrophon=
e and a solid substrate - a tree, for example? 'Blu tack', an elastic band =
to hold it close, gaffer tape, conducting jelly, Joe's Sticky Stuff? Thanks=
.