One morning, he wakes up on a faraway beach instead of his bed. He discovered his eyes have been replaced with biomechanical eyes that have forms like the headlights of a car, and they shine white light, barely visible during the day but noticeable in the dark and underwater. They are lined by a fluorescent cyan color, also illuminated.
His vision didn’t change, except that he can see perfectly underwater.
The lights only shine when he’s awake and conscious.
He can’t open his eyes all the way like most ponies.
These seem to be the only changes apparent, but there are a few other changes he’s not immediately aware of.
He doesn’t realize he’s a lot more drawn to the ocean than he used to be.
He can swim and dive despite never having learned, being a village pony.
He never really figures out what happened to him and it never happens again so he just lives like that now.