@theantlady I found this. I think they take a breath in to super oxygenate the blood and muscles then compress the lungs to prevent gas exchange.
And I was wrong about the time they can be submerged during an active deep dive. More like 20 minutes to an hour in full activity mode.
https://www.scientificamerican.com/article/how-do-deep-diving-sea-cr/
@semiotic_pirate
Right, 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.