Timeline: post "Captain America: Civil war".
Tony travels on his yacht, falls asleep and the yacht drifts near the island, where Peter lives. Peter is a merman, he grows tail in the water and has legs on the land. He escaped from New York after his parents (merfolk too) were killed.
The storm starts and Peter rescues drowning Tony, brings him to the island, tends to him in the cave. Peter doesn't tell Tony about being a merman.
At some point they get attacked by the pirates, who intend to demand a ransom for Tony. The pirates throw Peter overboard and Tony believes him to be dead.
Then Peter saves Tony from the pirates, they comandeer the pirates' yacht and go to New York. Tony brings Peter to May.
There was also an episode, when Tony rented an aquarium or a pool so him and Peter could swim there together.
Does any of that ring a bell?