Aquarion Water Co. plans to ask state regulators for an $88 million rate increase, a 42 percent jump, potentially raising the average household bill by more than $20 a month. The hike comes amid a legal battle over Aquarion’s proposed merger with the South Central Connecticut Regional Water Authority, which PURA recently rejected. Aquarion, owned by Eversource, says the increase is needed to cover infrastructure and operational costs not addressed in current rates. Attorney General William Tong and state lawmakers oppose the hike, citing excessive costs to customers.