http://www.sacred-texts.com/cla/numa/index.htm
http://www.sacred-texts.com/cla/numa/numa05.htm
From an essay written a century ago. Dated of course, but essentially correct. Carter does give evidence that as Mercury was introduced as "divine insurance" for the growing grain supply over land, Neptune was introduced for the same reason, only by sea - even if the Romans were not great sailors like the Greeks. I also think Augustus giving Neptune some credit for his military exploits against Antony and Sextus Pompeius is well taken...