You could also check Harbour Lake, which is across the street from Cypress Harbor. Grande Vista, Cypress, and Harbour Lake are the 3 closest MVCI properties. However, there are plenty of Marriott hotel properties also surrounding Sea World representing many of their brands including Renaissance, Courtyard, Residence Inn, and Fairfield Inn if you were looking for non-timeshare points/week exchange options.
As far as avoiding the actual walk to Sea World, if you stay along International Drive, you can take advantage of buying a cheap ticket on the Trolley that runs up and down International Drive. There is a stop at the front entrance of Grande Vista, but not sure where/how close the Sea World Trolley stop is.