The OP has a video showing the different effects of bouncing 3 balls - with different elastic properties - on water. There are slow motion videos of the bounces and underwater views of the cavities created by different balls.
Very interesting, particularly liked seeing the movement of the projectiles up the the wall of the cavity that they create in the underwater shots.
The location for this competition is a particularly attractive bit of Scotland - the island of Easdale. This is a bit surprising as the island (and the nearby village on what is almost the mainland, Ellenabeich) was once the location of a huge slate mining operation - presumably the source of all of the conveniently sized flat stones!