Bob,
Wouldn't you have to seal the ends for it to be waterproof? [Linked Image]

Is there anything that says that the wire in NM cable is THHN or THWN, or anything else for that matter?
We might see it as such, but actually it's only 90°C wire, a part of the cable, listed as NM.
I just "stripped out" a piece here, and the conductors aren't marked
Unless they're marked we have no way of knowing, and also technically we can't use it per 310.11(A) and (B)
...S