If fiberglass tub, you can go to a marine dealer and buy a fiberglass patch kit that includes all the stuff to repair that crack. Being an old Dolphin Sailboard enthusiasts in college, I had to make many repairs to my bow and I used just this stuff, it works great and when painted, it looks like new.
Then you could consider using JB WELD product, since its only a hairline. This would work, but I do not think its paintable, but you could repair the crack from the back and it will not show.
Then, you could use pure silicon caulking in clear color and it will work for a time and will need re applications.
Do not know about bondo. Don't know if its water proof, used this on metal doors with dings and it works great and is paintable.