though someone has probably already done it:
A giant computer operating on water pressure, hard-“wired” to do something like calculate and display epoch, or primes, or…?
It would be massive, couple of football fields or something. Maybe put it on a 45-degree incline. The idea would be that it replaces electrical current with water pressure in an intricate circuit of water pathways. It would be great if it was in fact run by gravity.