Hydrophones don't have to be placed underwater. Try placing one in the trunk of a tree visited by woodpeckers. Try burying it in the sand. Hydrophones can also be used for recording in air - particularly infrasound - when nothing else is available.
Waterproofing an existing microphone
You can experiment with underwater recording by waterproofing a microphone you already own. One way to waterproof a mic is to seal it inside a deflated balloon. For greater "sensitivity", try a latex condom (use an unlubricated one). Seal the open end around the mic cable, making sure to cover the XLR connector.
One way to do this is by making rings around the cable with electrical or duct tape (hint: WD-40 will remove any sticky tape residue). You could also construct a special use cable with a couple of rubber "O" rings around the mic cable.
Tie the condom tightly to the mic cable in several spots, stretching the condom over the rings. This method will work well to a reasonable depth (I've only tried it to 25 feet, the length of the mic cable).
I used a dynamic omnidirectional mic (the Beyer M58, to be exact, but any microphone will do). I have not tried this technique with electret or condenser microphones, only because I did not wish to risk an expensive mic in salt water.
You could also waterproof a microphone by dipping it in "Plasti Dip," a rubber epoxy used to rubberize handles on metal tools. The coating is flexible enough to transmit sound vibrations. The coating is water proof, but hard to remove.
The main disadvantage of the waterproofing techniques is the inefficient acoustic coupling of the mic element to the water. Your recordings might seem a little muffled. You may want to use a hydrophone, which is a specially designed microphone that transduces sounds propagating underwater.

Constructing a hydrophone
Typically, a hydrophone contains a piezo electric element, a preamp, and housing.
Take the piezo element out of its case and solder the leads to a cable. Waterproof this contact mic with "Plasti Dip."
Piezo elements can be found at Radio Shack, Mouser Electronics, or All Electronics. Edmunds Scientific may also have hydrophone elements for sale.
You could also attach (and waterproof) a commercial contact mic, such as the C-ducer, to a thin plate.
For more complex construction plans, Loughborough University has published plans for a homemade hydrophone and an associated preamp.

Commercially available hydrophones
Here are some lists of commercially available hydrophones with lots of technical information:
http://members.aol.com/jblue46498/hydrphns.htm
http://www.dal.ca/~whitelab/marmam/hp.htm
DPA microphones make a high end hydrophone.
Robb Nichols's PH1 hydrophone offers great value for the money.
You may also want to contact :
Offshore Acoustics
5454 Indian River Drive
North Vancouver, British Columbia
Canada V7G IL3
Phone: (604) 929 0440
Fax: (604) 929 0440
Email: jkford@interchange.ubc.ca
(website under construction)
Cost is about $350 USD.