The Poseidon desalination plant in Carlsbad — by far the San Diego region’s most expensive supply of water — is about to get even more costly.

Under pressure from state environmental regulators, the company is now scrambling to complete an estimated $159-million overhaul of its system for pulling ocean water from the Agua Hedionda Lagoon up to its $1-billion reverse-osmosis facility perched atop a nearby hill.