I'm sure we both mean the same thing. pH ranges from 0 to 14, with 7 being neutral. pHs less than 7 are acidic while pHs greater than 7 are alkaline (basic). So seawater is basic and could never, given the chemistry of eroding rocks and existing reefs, become acidic. You say it is becoming acidic - the data may suggest it could become less alkaline, I don't know.