What makes a submarine dive?

pumping air into its tanks to increase its density

adding water to its tanks to increase its density

pumping air into its tanks to decrease its density

adding water to its tanks to decrease its density

Letting water into its tanks, and letting it push air out, increases the density of the submarine. Once its density becomes greater than 1 (the density of the water it's floating in), it starts to sink.

The more water is let into the tanks, the greater the sub's density becomes, and the faster it sinks ... (it becomes more like a rock).