Not sure if I've done this one before. But anyway here goes. For anyone who is handy and or bored with nothin else to do.
A shark or indeed a submarine can be simply made out of an old pop bottle and a few bits and bobs. like so;-
For a shark make a nice dorsal fin out of a bit of cardboard, paint it black and glue it onto about the middle of the bottle.
The bath in your barthroom or a deep puddle can be put to use for the careful ballasting of the shark. Pour a little say cement into the bottle, just enough to keep the fin stably upright and to prevent it capsizing. when its set hard fill the bottle with water until it floats just below the surface with the fin sticking out, this is a very fine adjustment to get it right and anyone wanting to take a bath may have a long wait. Screw the top back on firmly.
Once ballasted with water as above make out of two bits of cardboard a pair of little hydroplanes and stick them on the sharks flanks angled down very slightly for forward direction submerging. If submarines are your thing just swop the dorsal fin for a conning tower and gun and paint it grey.
The big test;- Take your shark down to your boat and attach a long bit of string to its neck end and tie off the other to your stern dolly.
Now get under way and on tick over launch your shark/submarine gently over the stern paying out the string. When your boat has taken up the tow a steady V wave from your evil looking sharks fin or conning tower should be visible that follows implicitly every twist and turn you make. Speed up and its little hydroplanes if set carefully will cause it to dive, or the submarine to DIVE! DIVE! DIVE!!!. For those with a Klaxon hooter like me can signal the submarines dive. Slow your boat down again and the shark/sub will pop up and surface again. Do this again and again and again and again and again and again and again. You and everyone will be fascinated.