Making a toy submarine is a great craft project that will use recycled materials. Bottles are roughly submarine shaped already, so half the work is done before you begin. Best of all, you can unscrew the cap and fill the sub with water to make it dive like a real sub. Just be sure not to lose it at the bottom of a lake.
Get a bottle for the submarine body. Empty plastic 1-liter soda bottles and water bottles both have good shapes for a submarine.
Create fins. Cut two identical triangles out of a piece of plastic, such as the disposable plate for dive fins. You can cut a third triangle for a rudder. Glue the fins to the side and the rudder to the back. If this sub is going to be used in the bathtub, be sure to use a waterproof adhesive such as epoxy.
Create a conning tower. Cut a small hole in the top of the submarine and glue the long part of a bendy straw through it. The small end can be pivoted around to look in different directions.
Paint the submarine. Realistic submarines are dull, windowless gray machines, but your toy sub should have some flair. Paint it all in a bright color. Then, outline a row of windows in black. You can paint people smiling out the windows, or just paint the inside of the window frame a light blue.
If you put just enough water in the submarine, you can achieve buoyancy. It will float under the water without sinking or surfacing.