Container whale

At present, one of the best tools we have for monitoring underwater... stuff... is Spray (see linky), a wee submarine which operates on its own for long, long periods (it was last seen in the Jurassic period). This little beastie uses an inflatable bladder and some batteries(and a pump, and I dunno, some other bits and pieces) to periodically change its volume, making it alternately lighter than water, then heavier. As a result, it rises to the surface, then sinks down again, and using stubby little wings it converts this vertical motion into horizontal movement, crossing oceans with the inexhaustible patience (and approximate speed) of a big orange turtle.

Recently a couple of guys have made a cheap version of this sub (see second linky).

I'm thinking, why not big it up a bit and make it cargo-capable? Something considerably larger could be used for cargo that is not time-sensitive. The current little research versions show they can operate autonomously, and there's no real need for them to break the surface - they could alternate between a depth of, say, five metres and thirty five, which should be sufficient for them to get most places. When they reach their destination, they can log on to the mobile phone network and call up the local tug operator to come fetch them.

Without requirements for crew, these babies should be fairly cheap to make, and with very low fuel consumption could be an entertaining option. Fleets of them could traverse the oceans, carrying all manner of stuff. As long as you don't want to get the stuff there in a hurry, of course.

Of course, their near-silence and underwater sneakiness means there could be all manner of other uses for them...