I used the Magic Wand tool + Shift to select the white around the diver. I then used Select+Inverse and with the Move Tool I dragged the diver onto the sea.
I went to Layer -> Layer Mask -> Reveal All and used a black brush to erase away half of the diver. I then swapped to white and brought back a part of the diver at 20% opacity to make it look like she’s in the sea. I used the tool along the curve of a wave to make it look more realistic.