According to the account Peter Benchley tells in his nonfiction book Shark Trouble, he witnessed something like this during a Howard Hall expedition to the Sea of Cortez. During a swim call Howard Hall's wife Michelle saw a large manta with fishing gear jammed into the "shoulder" of its wing hovering just behind the boat; after inspecting it she got out, grabbed a tank, and the manta just hovered there while she pulled the ropes out. According to the account, after she finished it then swam off with her still hanging on and took her for a spin around the seamount ... before coming right back to the boat.
Where things get really crazy is the manta apparently hung around the boat for the next day or two and kept letting the crew take "rides" - it would hang behind the boat, someone would hop on, and it would take a roughly 30-minute tour before dropping them back off at the boat. I'm inclined to believe there's at least a grain of truth to the incident; Stan Waterman was filming it and The Girl of the Sea of Cortez - which was written by Benchley after that expedition and was inspired by that incident - has a shot of him on the back of the manta as the author photo.
I'm inclined to be skeptical of accounts of animals "asking" for help from humans or showing "gratitude," but if one goes by brain-to-body size ratio the manta ray is on par with a housecat. They have the highest brain-to-body size ratio of any elasmobranch. Whether that translates to comparable intelligence is open to debate.