Since the grains are not connected together, it's every grain for himself when they're dropped into the water.
The density of each grain is greater than the density of water, so each grain on its own sinks to the bottom.
The reason why a CUP or a BAG of rice has less density is because the grains don't all fit together perfectly, and 15% or 20% of the volume in the cup or bag is air, not rice grains.
"Rice is the perfect snack, when you're hungry for 2000 of something."
Mitch Hedberg
