Well, here's one of my ideas that I've had bouncing around for the Tech Contest. This is basically a miniature submarine, used for recon and transporting special ops over long distances underwater.
Since a manta ray's diet of planktonic food is fairly scattered, an alternate form of food was developed for the fabrication. Albeit foul-smelling, it does its job; many mantas can travel at high speeds for several hours before requiring feeding.
The food and other supplies are stored in a small compartment underneath the cockpit.
The pilot is also equipped with a set of filters that act as "lungs", successfully producing enough oxygen for them to be able to swim at a steady pace for about thirty minutes. (They were deemed impractical as a standalone device, though, as several men reported bouts of dizziness and nausea after that point.)
There's a few other thoughts I've had for this contest, one involving the trenches and a gas that renders the immune system useless... but really, all I've had so far are vague flashes of ideas. Any help would be appreciated.
Leviathan is aaall Scott Westerfeld's.