Grace, you are definitely not alone! I've certified many divers who thought they were too scared to ever actually dive. To be honest, when I first began diving I was a lot more scared than I expected, but now I train dive instructors in addition to divers! If you don't feel ready for the full Open Water course, most dive centers offer a "trial dive" called a Discover Scuba Dive experience. It's not a certification course but is a good way to get a feel for being underwater. However, don't write yourself off of doing the Open Water certification course- you practice diving in a pool or confined environment to become comfortable and confident with the equipment before you do any open water dives, so you will have lots of time to familiarize yourself with diving.
Whatever you do, just know that you are not alone! It's a common feeling to be scared, but with a good instructor you will be able to face your fears. Also, just so you know, even if a shark did see you underwater, he'd be a lot more scared of you, than you would be of it :) I've taken many people diving with sharks, and all of them are amazed and thrilled at how beautiful and timid they are!
Happy diving :)