Looking at it from the side (and from underneath, which you can't see in this shot), the damage extends 1/2" further than visible from above. I've marked in red felt tip the extent of the damage on the left. I pried a screw driver in there to check if there were any cracks further along the joint. Nope, it's strongly epoxied (?) together and that's the extent of the damage. Shouldn't be too hard to fix, even though the curves here will make it difficult to sand the new glass. It should be cosmetically fine -- as long as nobody pulls off the rubrail to look!
(PS, the newer P19's from International Marine have a totally differently designed hull and deck joint. The joint on my 1985 is flanged, the new ones overlap.)