They tie the prospective seals up, put a weight on them, and shove them in a pool. They're supposed to have to do a jump to get up and gasp for air once their breath starts to fail, but otherwise resist struggling. It's basically meant to weed out the sort of weakling scrub that would panic just because they've been given concrete shoes and made to sleep with the fishes.
She is either looking forward or completely oblivious of the fact she is about to be dropped into a pool without a way of swimming easily.
There's also the fact that she's a shipgirl - falling into the water from that high up will probably sting, but she'll probably float fairly easily (although orientation in the water could prove troublesome).
