What an epic experience! Lost count at how many Manta Rays we saw! They do barrel rolls right below you and swim so close that they brush up against you! The crew (Bryce, Rian, Avery) were attentive and educational. Bryce, the captain, was quite funny!
It does get cold in the water. So make sure the wetsuit jacket they give you is snug. The warm water shower on the boat was a godsend. Get a water proof cover for your iPhone or rent the GoPro for $40. They give you the microSD card. (I did this and it is worth it!).
It does get rolly out there so if sensitive, take something for the nausea.