Too late for you, but for others reading this: silicone was a poor choice. It's near impossible to get it all removed (even if it looks like it's gone) and nothing sticks to it.
Functionally fixing it is easy. Use a marine grade epoxy (not the putty stuff you find in the automotive aisle). You can use the fiberglass cloth from the auto aisle. READ THE DIRECTIONS. It's not hard but lots of people mess up the process. Google West System, they have awesome documentation and guides. Brush on a thin layer then lay out the cloth over the crack extending beyond the crack. Then brush on more until it is clear with no bubbles (it's messy and sticky so plan on how to keep it from getting everywhere). When it is cured, sand off any stray fibers sticking up (they will stab you otherwise)
Functionally, you can put it on the top but it will look ugly. If you can pull the tub and do it on the bottom it will look better but the crack will still be there with the layer underneath stopping the leak.
If you want it to look really good, you need to grind out and feather in the crack and then re-gell coat. Unless you are already good at fiber glassing, probably cheaper and easier to just buy a new tub and install it. The problem is it's thin to start with so getting a little crazy with the sanding and you can go right thru.
PS: This all assumes you provide better support or it will just repeat.