According to Intercot, the SeaCabs were suppose to be twice as long and simulate riding in a bubble. Check this out:
[I]"THE SEAS (Opening 1983). Visitors to The Seas pavilion will explore the wonders of the aquatic frontier through two major presentations. First, “The World of the Sea,” a ride-thru experience presenting various ocean environments; and second, “Sea Base Alpha,” a futuristic undersea research station complete with a 5,000,000-gallon tank supporting a living coral reef community.
In “The World of the Sea,” bubble-like vehicles will transport guests through a series of scenes depicting the visual drama of ocean kelp forests, abyssal canyons and other marvelous and mysterious ocean environments.
Guests will discover the complex interrelations of the physical and biological systems of the seas. They’ll also become aware of the finite nature of the oceans’ resources, and the vital need to coordinate human endeavors in ways that will ensure the stability of essential marine ecosystems.
Arriving at “Sea Base Alpha,” guests will disembark from their vehicles to explore a myriad of exhibits and participatory demonstrations. Observation modules within the huge tank will provide “aquanauts” with undersea windows onto the living coral reef community."[/I]
Apparently over half of the ride was cut to add the opening movie.