How do marine mammals control buoyancy?
From what I can tell, marine mammals can't dynamically control buoyancy during a dive. They ease the beginning of the dive by starting with a small lung volume to reduce buoyancy. 

Pinnipeds like seals do this by exhaling half their breath before diving.

Deep-diving whales actually breathe in before diving, but their lungs are small relative to body size to begin with. This makes their surface buoyancy weak enough to swim against without great difficulty (unlike humans).

As they dive their lungs compress rapidly with depth:

V = Vs / (1 + D/10)


V is lung volume, Vs is lung volume at surface, and D is depth in meters.

At 90m, the lungs are already at 10% original volume. Pretty soon they've shrunk enough that their effect on buoyancy is negligible compared to the density of the incompressible tissues. Presumably, these are close to neutrally buoyant (though the fact that dead whales sink suggests that they're slightly negatively buoyant).

Thanks to @souvik-bhattacharya for the ucsc.edu link.