Hello, and welcome to what may be the mode of transport of the future.
The possible mode of transport of the future is the amphibious, single wheeled, pedal powdered, submarine.
And below is a 3d model of it made in 123d design desktop, available to download here- http://www.123dapp.com/design .
Now if you want to know how i made read on ( and i recommend this) but if you just want the 3d model here it is - http://projects.123dapp.com/smb-123D_Design/monowheel-submarine/1783645 .
And don't forget to vote for the monowheel submarine in the 123d design contest if you think it should win.

Step 1: Starting Off

Firstly if you have not all ready download the 123d design software or use the online app.
Next draw a circle of any size, then draw another circle a little smaller than the first in the middle of it.
Now use the pull tool and pull the gap between the two circles outwards.
Then use the draw tool to draw 8 fins and pull them to the same height as rest of the model.
After that draw a square a little bit smaller than the inner circle in the center of the circle, this will be were the driver sits.
Next find the center of the square and draw a circle from the center of the square to the corner.
Finally pull the 4 outer parts of the circle you just made to the same height as the rest of the model.

Step 2: The Movement

The monowheel submarine will be human powered and will work in the same way as a bicycle, with a small wheel turning the larn one and another wheel turning that one.
Firstly draw a circle inbetween the inner circle and the center circle, like so.
Next pull the circle inwards but make sure they is an equal gap for the wheel ( i made the combined height 30cm with a gap of 2cm in the middle and the cylinders were 14cm each) and repeat on the opposite side and another 3 times on the remaining sides.
Now draw a line half way on the bottom part of the circle, then extrude to that point.
After that draw a circle from the bottom mid point and extend it to the inner circle and then pull the circle upwards 2cm ( so that it fits into the gap ).
Then pull the pull the part you extruded earlier back to its original size.
Next draw a diagonal line from the bottom left corner to the top right, now draw a rectangle on this line and pull it upwards a little.
Now draw a circle at the end of the line, pull it to the same width as the other circle.
Then draw a rectangle on each side as shown, then pull both of the rectangle outwards, next draw a line on the end of the rectangles and pull the square outwards.
Finally draw a rectangle from the center of the circle to the bottom.

Step 3: Possible Problems

To fix the possible problem of just spinning around i have add a stability tail.
To make it firstly draw a rectangle on each side, then a square on the end of each of them, next pull the square so that it touches the other side.
Finally add two triangle on the end of each side, to improve aerodynamics.

Another possible problem could be steering, to over come this a ballast tank could be add, so that if you wanted to turn right you would move the water to the right side of the tank.