From Wiki
The first aircraft to be painted in the "Shamu" scheme was N334SW (1988), a 737–300, and it was later followed by N507SW (Shamu II) and N501SW (Shamu III), both 737-500s. Subsequent to the retirement of Southwest's 737-200s, the 737-500s began to stay within a smaller geographic area formerly operated by the 737-200s, and as such, Sea World was no longer getting the optimal national exposure from these two aircraft. Two 737–700 aircraft, N713SW and N715SW, were repainted as the new Shamu aircraft, and both N501SW and N507SW were eventually repainted in canyon blue colors. All three current Shamu aircraft are no longer referred to as Shamu I, II, or III. The artwork on the nose of each aircraft simply states "Shamu". The overhead bins on the −700 series aircraft display ads for Sea World, except towards the front and back of the airplane, where the bins get smaller and are no longer uniform.