This is pure speculation on my part, and from a Californian no less, but...
The Seacab ride in The Living Seas has a 3.5 minute trip time. Disneyland's Submarine Voyage ride has a 9.5 minute trip time. It could be that the Seacab version of Finding Nemo will just have the one Mr. Ray show scene found in Disneyland's longer Finding Nemo Submarine Voyage version.
It's entirely possible that they pulled out the characters from the Disneyland concept art, which was released back in '05, on purpose because those characters won't be making the trip out to Florida. (Dory, Crush, Nemo, the Aussie sharks, etc., etc. are all missing from the Epcot artwork)
Mr. Ray and his class does appear in the WDW artwork, and they happen to be the characters that are going in to the first Disneyland show scene submarine passengers will see. Miceage and other west coast websites have stated that the new island and show building they are building in the Disneyland lagoon, effectively enlarging the show building that was already there, is being built purposely to house Mr. Ray and his class are the first Nemo characters who will introduce themselves to Disneyland submarine passengers and set the stage for the rest of the new Nemo show that plays itself out over the next five minutes inside the submarine caverns. (Check out LaughingPlace or Mouse Info or any other SoCal website for pics of this big new show building being constructed in the lagoon underneath the monorail beamway)
The Epcot Seacab ride might just include the Mr. Ray show scene from Disneyland, as a set up to the "learning" that will take place at Sea Base Alpha. And so, they purposely removed the images of all of the other Nemo movie characters from the Epcot artwork, simply because they won't be there in that Seacab ride.