In Mission 3, you are normally required to get the Walrus morph, and in doing so can enter the water on the boat in order to reach the Dolphin. However, if you stand on the right edge of the pool where the Dolphin is, and it happens to run next to you on the edge of the pool, because of its poor hitbox, you can engage in battle with the Dolphin. (Note that however the Dolphin's movement is still random so it might take some waiting.)