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A ship will float as long as it weighs less than the water it pushes out of way, or displaces. Ships can use materials in their hulls that are heavier than water, but there must be air within the ship. Since the air doesn't weigh as much as the water, this lowers the weight of the ship compared to the same volume of water.
Materials needed for the boat experiments
1. 20 paper clips
2. aluminum foil
3. ruler
4. bucket of water
Method
1. Cut 2 - 30cm squares from the aluminum foil.
2. Wrap one of the metal squares around 10 paper clips and squeeze the foil into a tight ball.
3. Fold the four edges of the second aluminum square up to make a small square pan.
4. Place 10 paperclips in the metal pan.
5. Set the metal pan on the water's surface in the bucket.
6. Place the metal ball on the water's surface.
Discussion
Why does the ball sink, and the boat float? They both weigh the same.
The boat floats because it displaces more water, and the boat shape hold air as well as the paper clips - and the air is very light. (A very simplistic explanation).
Alternative experiment
The only materials needed are modelling clay, and a bucket of water.
STEP 1 - Roll the clay into a ball and place it in the sink. Does it float? It shouldn't because the clay weighs more than the water.
STEP 2 - Now shape the clay into a rounded cup and put a small lump on the bottom as a keel. Put it in the water with the open part of the cup facing up so it stays filled with air. Does it sink? It shouldn't. As long as the bowl of the cup is large enough and contains air the clay will float. This is because the air makes the clay ship weigh less than the water it displaces.
STEP 3 - Now fill the bowl of the cup with water. Does it sink? It should. When the bowl is filled with water the clay ship weighs more than the water it displaces and sinks.