I see from the picture that you have an access port on the back of the daggerboard well. Can you access the cracked area from there? If you can, you should patch the area from the inside of the hull. Just sand around it a little bit, wet a patch of fiberglass and stuck it there.

If you can successfully do that, you're done. If you're anal you can fix the cosmetic inside the well by putting some coarse sand paper around a broomstick, sand the area, and then finishing up by spraying gelcoat + wax additive.

Now if there is a bulkhead and you can't access the area from inside the hull it becomes far more difficult to do a good repair. Access from the inside of the well is too tight to do a good job.