Because it gives them the opportunity to have a gratuitous beauty shot. Someone says "I want this shot!" and then they make it fit into the story (badly)
They should've stayed in orbit and used shuttlecraft. Since when were starship engines waterproof? And I doubt it would stay waterproof for long, especially being a ship constructed for the low pressure atmosphere of space, and then being put in high pressure under water... The more I look at these films the stupider they get.