I love the ferries so I did some research on the Southern Seas. The Southern Seas seen in this picture is actually the Southern Seas II. The first Southern Seas sailed Seven Seas Lagoon from 1971 (pre-opening) to 1975 when it was damaged in dry dock. Parts of the original Southern Seas were integrated into the new larger Southern Seas II.
Southern Seas II was retired from daily service in the early 1980’s but was used for Mmdnight cruises and Discovery Island tours until 1996. Pieces of both Seven Seas can be found at Typhoon Lagoon. Look for her bell, gauges, and steering wheel amongst the rubble.