First, why use a toothbrush? I have found that toothbrushes are great waterproof motors. I have tested them so far to depths greater than 5 feet with no leaks. Most other store bought submarine motors leak at these depths and eventually rust up.
Step 1: Obtain Materials
1.Duct tape(shiny silver duct tape is better, but it is more expensive)
2.Anything buoyant enough to float your toothbrush, I used a Bawls bottle, but you could use anything else, I have even used a plastic bag.
3.Colgate motion toothbrush
4.Half an easter egg, mine was 3.4 cm(1 1/4 inches) in diameter and fit the toothbrush perfectly
6.Lego axle 3 studs long.
7.Lego axle connector
8.Alignment tool 10 stud long axle and what I like to call an i-piece
9. Cowling, Axle 3 studs long, i piece, and cowling piece.(Optional, cowling piece helps prevent the propeller from undergoing "spontaneous mechanical failure" if it hits a wall)
10.Disposable lithium batteries(optional, but they will make your torpedo go much faster)
11.Hot glue gun with hot glue