Ugh that's the worst feeling. Been there with those miracle-in-a-tube products. For a real hull patch in saltwater, you need a two-part marine epoxy putty, not the single-stuff. Something like Marine-Tex or Pettit Water-Patch. The key is prepping the area like crazy, sanding it down to bare, clean fiberglass before you even open the tube. Let it cure fully, way longer than the package says, before it touches water.
My buddy learned this the hard way last season. He tried a quick fix from a tube on a small crack near the waterline. It looked fine in the yard, but the first time he launched, it just peeled right off like a bad sticker. Had to haul the boat back out immediately. He ended up doing exactly what you said, sanding it to clean glass and using the two part stuff. That patch is still holding now.