large sea turtle
Zoological term, specifically referring to the loggerhead sea turtle (Caretta caretta) or similar large sea turtles. Cross-referenced with 青海亀 (あおうみがめ).
See also: アオウミガメ (あおうみがめ)
The 正覚坊 is a large sea turtle that mainly inhabits warm seas.
Slang or dialectal usage, possibly from the image of a turtle drinking heavily. Rare and likely regional.
He is such a heavy drinker that they call him 正覚坊.