I know of that quote - I mentioned it an age ago in this thread - and it is simply wrong. If you read what Caesar says in the Bello Gallico, you will understand that the temple of Nehalennia must have been smack in the middle of Menapian territory.
Here's a map of their territory (and I assume you know that the temple of Nehalennia was located on the islands you see in their territory):
What evidence do you have that the Menapii were sailors? Where are you finding this?
The area of Nahellenia may have been situated there (at the mouth of the Rhine) but possibly the Menapii, if they were not sailors, did not worship her OR because we do not know the Menapii worshipped her it has been left out of any reference.
Edited by The Puzzler, 06 October 2012 - 04:33 AM.