Some of the state’s biggest water districts are about to make their opening moves in a financial chess game that ultimately could saddle the Southland with much of the bill for re-engineering the failing heart of California’s water system. In coming weeks, the districts are expected to decide if they want to sign on to California WaterFix — a long-planned proposal to construct two massive tunnels that would change the way water supplies move through the Sacramento-San Joaquin Delta.