Is it normal or mentally ill for a child to watch Jaws and be scared that the shark is under the coach and will appear and bite their feet ?

Horror movies are just for fun like reading horror novels, playing horror games, watching horror game streaming, etc. Children sometimes want entertainment and enjoyment but fun is not for everyone (e.g., if most mathematician or who likes math that wants solving math problem but he/she doesn't like playground because of his/her opinion such as boredom or scared). 

Jump scare is normal because it surprises the fear but not normal on boredom or traumatize/horrified. Just like imagination. You know that trick or treat and cosplay exist right? Also, it can be effective way to cure hiccups.

It's normal...but what's not normal is an adult letting a child watch scary movies. Now that's something mentally ill.

Children should not be watching scary movies.  A good parent will guard the eyes and ears of their children.  They should teach their children that falsehood prevails in movies and movies do not always provide truth.  A young mind is a fragile thing and it is up to the parents to protect their child.

no, make him seek mental help or use electric therapy 

If a shark is under the coach being pleasured, then I don't think the shark will be interested in attacking some kid.

Craig T. Nelson is a republican, so I can see how Coach would be scary. If little Timmy is afraid of images flashing on a box, he is in for a long, wussy life.

I would think it is normal. Did you mean the couch? I know children are often worried about monsters under the bed, or in the closet.

It's perfectly normal. I watched Jaws as a child and I remember checking behind my bedroom door afterwards as I was so scared. 

Its quite normal for a child to become afraid of something under the couch (I doubt you have a coach) or the bed or in the closet.

There is a reason they don't recommend scary movies for children.

Very normal.  Children often can't separate fiction from reality.  And they don't yet know how or where sharks live.  That's why they shouldn't be allowed to watch scary movies.  Heck, with its eerie music and sudden shark pop-ups, even some adults became scared of going into the water after watching it! 

Jaws is rated PG for just that reason.  PG means "Parental Guidance Advised".  If in doubt about a movie, just do a web search naming the movie and adding "parental rating".