Floating a mattress on water may seem like a peculiar idea, but it is a common question people ask. The answer, however, is not straightforward and depends on the type of mattress.

An innerspring mattress, for instance, is unlikely to float on water. This is because of its metal coils that make it heavy and dense. Therefore, an innerspring mattress will sink to the bottom of the water if one attempts to float it.

On the other hand, a foam mattress or a waterbed mattress can float to some extent. Foam mattresses are made from lightweight materials that can trap air, making them buoyant. However, the level of buoyancy varies based on the density of the foam.

Waterbed mattresses are filled with water, making them buoyant. But, they are not designed to float on top of the water. Instead, they require support from a frame or base to prevent them from sinking to the bottom of the bed.

Nevertheless, even if a mattress can float on water, it may not be safe to do so. The wind can blow it away, or the current can carry it away, putting one's safety at risk. Hence, floating a mattress on water should be done with caution.

In conclusion, while some mattresses can float, it is unlikely for an innerspring mattress to do so. If you wish to float a mattress on water, a foam mattress or a waterbed mattress could be an option. However, always keep safety in mind when attempting such a feat.