Complex sentence: Sharkey usually keeps the company of Seaman Kowalski when on shore leave, which Kowalski who once recounted three days they spent in San Diego on what Kowalski referred at that time as a "binge".
Split into simpler sentences: Sharkey usually keeps the company of Seaman Kowalski when on shore leave. While on shore leave, Sharkey is known to spend that time with Seaman Kowalski who once recounted three days they spent in San Diego on what Kowalski referred at that time as a "binge".