You don't, but it's always best to reserve just to ensure your entrance. It just a safety measure on your end, it was needed back when COVID first started and they had to limit the number of people, since then I haven't noticed any restrictions. But, isle du boi gets pretty busy, so if you can reserve, I would!
Last two weekends they were turning away those without day passes due to park being at capacity
No need for reservation to swim or visit.
Thanks! Your answer is awaiting moderation.