Is that a keel bolt on the other side of the thing you're calling a stringer? Is there (or should there have been) another bold inside that stringer? Given its location, it seems to me like that might be along the keel/hull joint.
Short term solution: solar panel, battery, and 12v pump that can run dry; just let it run continuously. You could even be proactive and put a tube into the hole in the stringer.
Longer term solution: get her to a yard so you can have her hauled and inspect that area more carefully. If that isn't an option, try a camera like this:
Inspection Camera - Save on this Digital Inspection Camera
or, if that lens is too big to fit, this might work:
Digital Inspection Camera with Recorder
Another option is to get a small pencil-sized flashlight, tie a string to it, and use that to light up the inside of the stringer. Or drop a glow stick in there.
Unfortunately, in the end, I suspect you're going to wind up having to haul her unless someone knows of something hidden under that stringer.