Complex sentence: Megan does not realize that Jonathon has gotten out of the water, and feels hands touching her, assuming it's Jonathon, until seeing him on shore, in which she begins to panic and swims to safety.
Split into simpler sentences: Megan does not realize that Jonathon has got out of the water, and feels hands touching her. She assumes it's Jonathon, until she sees him on shore, upon which she panics and swims to safety.