I used a piece of styrofoam about 1/2 inch thick (See the right portion of the graphic.) and a plastic canister from a roll of 35 mm film. Not shown is a piece of clear plastic tube for a fish tank air pump about 3/16 inch in diameter on the outside or less.
Cut out the styrofoam to receive, but still support the film canister. The hole at the rear of the boat is for the clear plastic tube. Shape the styrofoam to resemble a boat hull.
Step 1: Inside the Film Canister
The left portion of the graphic shows a cutaway view of the inside of the canister with the wire support in place. The right portion of the graphic shows the wire support.
Step 2: Just Add Water and an Alka Seltzer
At the right half of the graphic you see the film canister cap with clear plastic tube inserted through a hole drilled in the cap. The tube should fit snugly in the cap. The tube should also be a bit longer than shown here.
Without allowing the water to come into contact with the Alka Seltzer tablet yet, place the cap firmly onto the canister.
Step 3: Use the Boat
I would like to say my daughter won the challenge at her school. She was too young and small to insert the Alka Seltzer reliably after arriving at school. So, she had to walk about three blocks to school and the water in the canister came into contact with the tablet before she arrived. But, it worked well when we tested it at home. She is an adult now. But, she wrote me a nice thank you on tablet paper, which note I still treasure.