The diver sinks to the bottom because of gravity. You can move him left or right using the arrow keys, and this keeps him from sinking because the script contains counter gravity code.
3D effect. The backdrop is vector, so it is easy to make the foreground rocks and seaweed into a sprite. Then bring to front with layer code and watch the diver disappear behind the rocks. The vector eraser tool makes it possible to create costumes to add an element of animation to the diver which has only one costume in the sprite library. Try adding a bubble effect as in the submarine at https://scratch.mit.edu/projects/524919814
