Explain to them how it's consuming a lot of water and explain why that's a problem (plants, other animals, need water etc). Then challenge them to come up with

Explain to them how it's consuming a lot of water and explain why that's a problem (plants, other animals, need water etc). Then challenge them to come up with a way to solve the problem. Autistic kids are often incredibly good at solving practical problems like that and it'll give them a better understanding of how things work. Faucet heads that are designed for saving water often are set-up to create the "feeling" of lots of water, without actually using the same volume (water saving rain style shower heads are particularly good at this). Something like that would probably preserve any sensory elements of the experience they enjoy and would cut the water consumption by 60-70%.