Complex sentence: Since he was young, he's been taught by his parents to fear water; they don't want him near it because his true parents would be able to locate him, not to mention certain changes happen to Jackson when he gets in the water.
Split into simpler sentences: Since he was young, he's been taught by his parents to fear water... they don't want him near it because his true parents would be able to locate him. Not to mention certain changes happen to Jackson when he gets in the water.