Gregory's ServerRight, so, the general thinking is deep diving mammals spend a period at the surface engaging in gas exchange to replenish oxygen stores in tissues (and remove carbon dioxide). Then there isn't an advantage, and if anything, there's a disadvantage, to filling the lungs with air at the start of the dive.
@semiotic_pirate
I think this detail has stuck with me because it's surprising compared to how humans typically dive. But we don't go very deep (without scuba gear!) and we don't have as dramatic of circulatory changes as the marine mammals do.