What is density of an object in water?

Answer 1

The same as when it is in air, unless the object interacts with either water or air, or both (in different ways).

Density is dependant on the object's mass and volume and has nothing to do with the medium in which it is sitting, unless that medium interacts with it in some way.

Answer 2

Provided that the object does not

• dissolve in water (to form a mixture),
• chemically react with water (as sodium does),
• absorb water (as a sponge does), or
  
• get compressed by water pressure,

its density will be the same as its density anywhere.