Expanded lungs?

You've got the right idea. The density of sea water is about 1000 kg/m^3 and g is about 10 m/s^2, so density*g*h is about 50,000 Pa. Atmospheric pressure is about 100,000 Pa, so in surfacing, the diver goes from 150 kPa to 100 kPa for a reduction in pressure by a factor of 2/3. PV=nRT=some constant (since temperature is constant), so you want PV=P'V' where the ' indicates after surfacing and the regular variables are before surfacing. The previous calculation showed that P'=2/3P, so PV=2/3PV', or V'=3/2V=1.5V