I did a scarf joint on the ends of my gunwales since my stock was 16' nd I wanted to go longer. I used epoxy and pegged them as well. I never read anything on the long-term water resistance of poly, but epoxy is tried and true and seems to hold up well with a lot of moisture exposure. (5 to 1 and 6 to 1 epoxies are actually best for long term moisture exposure). I used regular 2 to one. Those areas between the skin and the wood can stay moist for a long, long time.

If you put a straight edge on a gunwale of a S & G boat, you find that the bend is not equal but much more in the center. If your repair is on the ends, don't even worry about the flexability.