Now this isn't an attempt at grappling or wrestling in the combative sense. There are far better sources than myself to go to for that. This is a game/interaction to allow people to sincerely explore and roughhouse in a safe space.
I picked this up from Tom Weksler at his Movement Archery workshop: the task is to try and take your partner down to the ground but you don’t want them to take you to the ground. Once you both arrive on the ground, you then want to lift your partner off of the ground and again avoid them lifting you. It is about control and intent - who decides who goes where and not necessarily in a combative context (so generally I would suggest to not try any takedowns, judo throws etc... save that for the gym/dojo).
It is intended to be playful and tactical, using cunning over brawn is one way, but if it evolves to be a little rougher due to the rapport you and your partner builds, that is ok too. It is a loose guideline to explore. As you can see from the video, it's also fun and possible to play with odd numbers - one person floats on the outside and then interrupts the dance of a pair, for the person who is left out to float and continue by interrupting another pair and so on. Why underwater? It's just some imagery to help stay relaxed and fluid.
To paraphrase Tom: try and find "the space between combat and expression, free of gestures".