Drowning scenes

I'm just curious...in the two drowning scenes of the crews, were those actually filmed under water? It looks so realistic, yet I don't see any bubbles or anything coming out of their mouths. Their hair, Lt. Dixon's wife's clothes...all look like they are under water and their faces look whitish like they're under water, but their facial expressions don't look like they're holding their breath. It's driving me crazy trying to figure it out!