The first time we launched our boat several years ago, our boat a GRP Mk1 took on water. We eventualy found a crack in the centre board case, the crack was on the join where the two halves of the grp centre board case had been joined. I suspect the previous owner had jammed maybe a stone in the case and lifting the centre board up had forced the stone to the front of the case and split the seam.
To find the leak I filled the boat with water and looked underneath to see where water was leaking out.