The vast majority of the time: If you have a joint between different materials, you expect to get such incidental "cracks". In reality they were not "jointed" on VERY proximate with some sealant deeper doing it's job.

UNLESS there is something more, if it were me, I would document the condition (photos with some scale object -- and whatever makes you happy) then I'd watch it. I believe its just a cosmetic issue. Some FLEXIBLE (but not intrusive) caulk that PAINT will stick to would work.

I wouldn't WIDEN the crack and try to inject sealant into it. I'd leave well enough alone.

If you believe that the crack is a result of keelbolts that are loose (or that the boat hit a ledge, or something like that) -- then you really are into the proper fix being re-bedding the keel. Again, I wouldn't do that unless your really had some very good reason. If you did it, a few years from now, you'll see a crack again.

Of course, you want to look and see if the fiberglass stringer system shows any distress -- or if there are any signs of such things. But I really doubt that if your surveyor did his job. Absent something like that, you should be fine.