There were two of these Excursion Steamers in the early fleet and unlike the one on the pre-opening card neither of them was named the Osceola. They were named the Southern Seas, and the Ports O' Call. and in the early "pictorial souvenir" books it was often noted that they "have "walking beam" engines unlike any built in 100 years" Even though in the end none of the steamers ended up being named Osceola they were called "Osceola Class" Here's Captain Matt with more: There were actually two side-wheel steamers named the "Southern Seas" The original "Southern Seas" was retired  in 1975 and scrapped in 1977. The "Ports of Call" was retired in 1982 and scrapped in 1984. The second "Southern Seas" was built in 1977 after the scrapping of the first. After the opening of Epcot in the 1980s the boat wasn't used as much. It was used for midnight cruises, and for trips to and from the World Cruise dock to Discovery Island. But in 1997, it was deemed unnecessary and was taken out to a hill behind dry dock and scrapped after 20 years of service."
Also in closer views of these "Excursion Steamers" it looks like they have a much smaller capacity than the ferryboats. Still according to one source in the early days of the park they were so pressed for capacity in getting people to the park they were for a time pressed into service. David Koenig writes "Although they would eventually be used for "pleasure cruises," their initial use, starting with Opening Day, was to supplement the Monorail as transportation from the Transportation Center to the Magic Kingdom, until other ships and Monorail cars could be added." In reference to the pleasure cruises David mentions here's a description from one of the park "Information guides" :
"WORLD CRUISE. Sail the 650-acre Waters of the World aboard a colorful side wheel steamboat. Narrated tours describe Walt Disney Worlds history . . . and preview it's future. adults $1. Children (3-11), 50¢." (winter/Spring 1974 information guide compliments of GAF). In the resort section of the same guide The Polynesian, Contemporary, and Fort Wilderness resorts all list under Entertainment: "Moonlight Cruises aboard colorful side wheel steamers depart the"Polynesian Village Marina/ Contemporary Resort Marina/ Fort Wilderness Landing " for a full evening of fun afloat. Cocktails and soft drinks are sold on board." I also have a Summer/ Fall 1974 information guide in which the resort information on cruises is basically the same but in which the attraction listing for the World Cruise is limited to basically being listed as one of the possible ways to get to "TREASURE ISLAND!". Treasure Island was the Early name of Discovery Island and according to Disney A to Z it opened April 8, 1974 and changed its name to Discovery Island in 1977.

A VOYAGE ON THE PORTS O' CALL THROUGH THE EYES OF A LIVE STEAM ENTHUSIAST Hi Brian, Here is some more information on Ports of Call. To those of us who are steamship fans, these vessels were a rare treat, and are sorely missed. Both of these vessels were rare, modern replicas of the "walking beam" paddle steamer. The walking beam engine was unique to North America, and at one time was the most common type in ocean and deep fresh water. The last operating original walking beam steamer was the San Francisco ferry Eureka, which stopped operating in 1958. I rode the Ports of Call as a little boy in 1979 or 1980. I knew about it in advance and specifically made the effort to plan our schedule. I spent most of the voyage standing at the engine room door, watching the engineer. Everything was as I expected. The most fascinating part of the engine is watching the engineer get under way. The walking beam engine has a single cylinder, like a lawn mower engine, BUT there is no such thing as an electric starter. The engine has to start itself by careful steam application. The engineer controls the steam with a "starting bar", a big lever attached to the valves. He watches the crankshaft and swings the bar up and down to shift the steam to each end of the cylinder. This bar was big, as tall as the engineer, and he did it while standing, if I recall. He had to keep doing this until the engine was rotating smoothly and then he could engage an automatic valve gear. When stopping he had to reverse the process, and make sure the engine stopped in a position that allowed starting. If he failed, the engine would be stuck on "dead center" and powerless. In the old days they had to pry the engine off center with a big timber pryed against the paddlewheel. Perhaps Disney had some jacking gear built into the ship. The trains and Liberty Belle boat do not have this problem because their engines have two cylinders synchronized on opposite cycles. The whole experience was very unique and realistic. You could really tell the engine was authentic. The whole vessel squeaked and warped with the engine's movement, because there was only one cylinder you could feel the power surge with every stroke, similar to the surging of a scull when the rowers are synchronized. -- Steven Harrod (LINK TO EMAIL)