Step 2: Making the diver
For mine, I cut the sides off of a rubber bouncy ball so that it became a disc shape, then carved it into a heart. I tested this in a glass of water and it floated no problem, so I needed to add a bit of weight.
I then got a felt-tip pen lid of a similar colour and cut it down into a tiny bucket, and attached the two with blu-tack and the shaft of a cotton bud, making a kind of 'heart air-balloon'. After a long time removing air bubbles in the blue tack by pushing it down with tweezers, and adding/removing miniscule amounts of blue tack to make it heavy enough, I finally achieved a workable object. However, I found the blu-tack coming apart in the water and it stopped working, so I replaced most of it with hot glue, and it works fine.
When making your own, it is important to keep testing it in a glass of water, so you can tell if it floats too easily or sinks. Once you think you have a working object, try it in a plastic water bottle, as then it is easily removable and testable (you'll see why testing with a glass bottle should be left until the end in the next step). It is also quite important to make sure your diver fits through the neck of the bottle! If you are confident enough, you can construct it inside the bottle, but you'll need a wide-enough neck to fit your tools through.
I recommend leaving it for a few days and playing with it to make sure it works over time!