On the Big Island. We did it with a tour group at night. Lights from hotels and the boat attract plankton which attracts the mantas. It was my favorite part of my Hawaii trip.
I am in Hawaii and I have been very diligent about putting on sunscreen. But, we went looking for sea turtles and found some so I quickly hopped out of the car to look at them and ended up standing their for awhile watching them. Alas, my feet suffered because I had flip flops on and hadn't put sunscreen on my feet yet. -Click-