Hi, I'm the OP.
The unit is a Sea Breeze SMZ42H46ZOGX.
The instruction manual and even the promo literature explicitly calls out a 6AWG Stranded 3 Conductor cable, with a 50amp breaker.

Weird thing is that of the 3 conductors, it appears to send one to ground (http://i899.photobucket.com/albums/a...s3e1fd5ca.jpg). And since I have a "6-3 Romex NM-B with ground", I now seem to have an extra (unused) wire... the neutral.

Do you agree?