I have used a line of credit on NCL in the past, so I guess I can chime in.
1) There is no fee for using markers.
2) The nuts and bolts of using your line of credit:
Hand your cruise card to the dealer and ask for $X of credit. The pit boss will pull your info, then authorize the dealer to give you chips. A few minutes later, the pit boss will present you with a marker to sign. This is, in effect, a check you are writing to NCL, which they agree to sit on for the duration of the cruise.
At the end of the cruise, you are expected to “settle up” your outstanding markers at the cage, buying them back with your extensive cash winnings, or, more likely, by writing them a standard check in exchange for the physical markers. If you fail to or forget to settle up, NCL will deposit the markers against the checking account used to establish the line of credit.
3) You’d likely each need your own line. If you typically gamble together, you could have just one of you with a line to buy chips, and then you all could divide them however you choose.
4) You certainly do not need to use your entire line. I do suspect they would get very irate if you were taking out markers for $20 or even $100, due to the paperwork involved. I would suggest $500 or $1000 would be appropriate increments.
5) One con that I experienced was that I lost track of how many markers I had taken out, so I was short $1000 when I went to settle up. Now, the markers do clearly indicate the table and time of issue, so they were able to jar my memory that I did, in fact, take out a marker at 130 AM at the blackjack table. “Oh! I’m the idiot!”
I now always take a photo of each marker immediately after signing it, to avoid future embarrassment. Your mileage may vary on that point, depending on your drink package usage. 🙂