What it sounds like is you can fill the crack with non-crosslinked PE to stop the water from entering, but you will not be able to bond this filler to the outer "shell" which is the crosslinked PE. If the sublayers are liner PE (which I think they are) you can bond this filler to them. The filler should be the same material so it will melt togeather the filler and the boat.

As Matt Miller alluded, you are getting into the realm of chemistry when trying to repair this type of material. The repairability of the hulls very much depends on the process and the material used. Not all polyethylene is the same. Even the same PE can be different depending on how it was molded.