Swim times are allocated at check-in. They will merge your group with other guests until they have 8-10 participants in each group. You don't get to select your time, it is assigned. Generally speaking the earlier you check in for your all-inclusive day, the earlier your dolphin swim time will be.
In the summer, especially, those early times are the ones you want to get to ensure you get your dolphin swim in before the afternoon thunderstorms cause delays.