There are a handful of weird and cool wildlife experiences that all intrepid travelers should seek out. Watching Mexican free-tailed bats emerge in the evening from their roost from under the Congress Avenue bridge in Austin, Texas is one, and swimming at night with the manta rays in Hawaii is another. I'll remember this for all my days.
This is not a wilderness experience, but (much like the Austin bats) is an amazing, yet accidental convergence of wild creatures and the built environment. Many years ago, the Sheraton hotel decided it would shine flood lights into the water at night so land-lubbing guests could peer into the ocean and observe fish from the hotel's restaurant. Lo and behold, manta rays started showing up to feed upon zooplankton that were attracted to the light. Fast forward to today, a bunch of boats take groups offshore, plop you into the water with a herd of 60 others, shine lights into the water, and wait for the manta to arrive.
You're not really swimming in this endeavor...instead you and your group hang on to a floating platform thingy and you kinda just float there with your snorkel and mask peering down into the water. LED lights shine down from the platform attracting zooplankton and the rays. The massive black and white creatures rise from near the ocean floor in wide, graceful arcs with their mouths agape filtering zooplankton from the water column as they swim. We were lucky enough to have several of the manta come within a foot of our masks as they glided along. So freaking cool.
Sea Quest did a great job and I highly recommend their charter. Crew did a great job narrating the experience by speaking about the whole hotel/lights backstory as well as the life history of the manta rays. They were deeply respectful and thoughtful about the conservation of these wild animals. Tip...wear the wetsuits that Sea Quest provide; the water is slightly chilly and the wetsuits keep you comfortable in the water.