Definitely shorts/swimsuit (and make sure you apply suntan lotion before your tour). I wouldn't even consider bringing a pair of jeans with me to Aruba (too hot!). Besides, you would not want to waste time changing in/out of your bathing suit when you got to the Natural Pool (i.e. less time spent at/in the pool).
I would also advise bringing water shoes to the pool (it is a little rocky/slippery in/around the pool). However, you should also have a good pair of shoes (i.e. running shoes - not sandals/water shoes) for riding your ATV (safer) and for walking down to the pool itself (many stairs and rocks to climb over to access the pool).
P.S. Do not forget to wear a pair of sunglasses on your tour (it is very sunny and dusty on the trails).
P.P.S. (for Arubalisa). I did the Kini Kini tour and it was very good.
