I'm glad your took it that way after my explanation, in opposite to the last sentence of your initial response which I found a bit agressive :)

I made the mistake of not checking the exact english terms. In my language we differentiate density and relative density with two different terms, litteraly volumic mass and density. I thought op wanted the relative density

In deed, you should try to press the sponge to evacuate all the air trapped inside.

Density is an absolute ratio, where the volumic mass of the water is the reference (water density =1). By dividing the volumic mass of the sponge by the water's you compare both, and will easily know if it's denser (desnity > 1) than water or not.

Take you dry sponge, put it on a scale, you have its mass. Take a graduated glass recipient with exactly 1L of water in it, put the sponge in it, read the new volume indicated, substract 1L to it, you have the volume of your sponge. Divide the mass by the volume and you have the volumic mass density of the sponge. Divide it again by the volumic mass of pure water (1000kg/m3 or 1kg/L) and you obtain its density.

Edit: corrected as I didn't get the correct english terms.