As the balloon goes down, it's compressed more and more. If you hold it at any depth, the pressure of the gas must match the pressure outside. If it didn't the balloon would expand or be compressed until it does. Thus, as the balloon goes deeper, its density increases because the mass of the gas is constant while the volume decreases. There is a specific depth at which the pressure is such that the density of the gas matches the density of seawater. Say that with an equation and remember to use subscripts.