Wholesale water rates in San Diego County — a key factor in how much local residents and businesses pay for water — will rise next year by less than half of what officials were predicting last winter: 8.3% instead of 18%. But the Jan. 1 increase, which the county water authority’s board of directors approved Thursday after months of debate and negotiation, is still a substantial hike that brings the cumulative two-year increase to 23.1%.