First, drill a small hole at each end of the crack. This will help stop the crack from propagating.
Next find some sacrificial plastic similar to that in your hull. I used an old kayak seat. Cut out a piece large enough to cover the crack plus a good area all the way around the crack - at least half an inch or more in every direction.
Most likely the area you're trying to fix is curved in at least one dimension. Heat up your piece of plastic using a heat gun, oven, etc. until it becomes plyable, then (protect your hands!) press it onto/into the area over the crack so that it form-fits perfectly over the target area. You'll probably have to repeat this heat-and-press cycle a few times to get the shape just right.
Smooth the edges of the patch (this is easier before it's epoxied to the hull!). Roughen the side to be epoxied with sandpaper. Do the same to the target area on the hull.
Finally, use two-part epoxy to attach it to the hull. Apply pressure (clamps, whatever) to insure a tight fit while the epoxy cures. If you had to cover a hole (for a bolt, etc.) drill it out after the epoxy is completely cured; to keep things symmetrical you may need to add a washer, etc. to allow for the additional thickness of the plastic.
Once it's cured you'll have stopped the crack from propagating, distributed whatever force caused the crack over a much larger surface area, AND reinforced the hull in that region so it will be stronger than before.
Hope this helps!
