The quills are very light and act as a life preserver which helps them float when they swim. The quills also offer a cushion if the porcupine were to fall from a tree.
All fruits do not float in water. Some fruits float due to their peel, like lemons or oranges, which makes them less dense than the surface tension of the water.
The answer to your question depends on several factors. For example, it depends on the shape of the plastic item. If one were to be permitted to mold the plastic into the shape of a boat, then the...
A substance will float on water or not depends upon whether its density is greater than density of water or not. If it is greater then it sinks otherwise it floats.