I used to attach a fishing line to a bolt and draw it up through the hole on which I could fit a grommet, washer and nut. To accomplish this you will have to lower a for instance 5 mm nail through the hole to about 1- 2 meters beneath the hull. Press a cork in the hole to stop the flooding and clamp the fishing line. Retrieve the nail and line with a boat hook or trawl for it.
Prepare a suitably sized bolt with a tapped hole at the thread’s end and insert a threaded pin with a ring at the top which must be fastened to the retrieved fishing line with a washer and grommet against the head of the bolt.
Drop the line and bolt and pull it up through the hole. The bolt will shoot into the hole with a clang and will probably be held in place by the hydrostatic water pressure. Remove the fishing line and ring insert and put on a grommet with washer, screw on a nut and tighten. Job done, leak fixed.