Have you watched deep space 9? Your dad might be correct, a symbiote might not kill Matthew, though it would likely change him. In the series, the trill species has a relationship with a worm-like symbiotic species, though they generally fuse as adults, not children. The host's personality combines with the symbiote's personality, instead of erasing it.

I'd also recommend reading the Parasitology trilogy by Mira Grant (sorry for the offensive name). Depending on how it works, maybe you could find a way to lay eggs in humans with unrecoverable brain damage. That way your species could survive without erasing a person, if it turns out that's what happens.

Maybe start or join a support group for people afraid of water and try to get other information from people like you.

Either way, I think you're a great dad and a good person, good luck :)