A shipboard credit is not part of the cruise fare so it won't be on your invoice. It should show up on your "sail and sign" account after you check-in and activate your account. At least that has been my experience.
Yes, be sure to have any evidence that you have an onboard credit due to you. That way if you don't see it on your account (You can check at the pursurer's desk or lookup you account info on the TV in your cabin) you can go to the pursurers desk and they should be able to put it on you account. If you don't have the paper work showing you should get it, you will be out of luck.
This may have been answered, but for clarification..If the On Board Credit is a booking incentive it will not show up on the documents. It will be reflected on your Sail & Sign account. If it is an On Board Credit for a fare adjustment it will be noted on the documents.
Also, if it's on your confirmation from your agent as you mentioned you're safe. I don't know exactly how the agency you booked with works but where I am there is a little box we click to add such credits. Then another to pay for them if they're an agency promotion rather then a cruiseline promo. I've actually missed clicking the little pay for it box before and had a client not get the credit they were supposed to recieve. In such human error cases a phone call to your agent will result in a check from the agency when the error is reported. Either way, if you have a confirmation showing you're entitled to it, you're fine.
The shipboard credit is, in fact, a TA booking incentive. Like everyone has correctly informed me here, the TA has also confirmed the credit will be added to our Sail & Sign account. I feel more relieved now. But to be on the safe side, I will definitely bring the TA contract documents listing the credit, as well as the email confirmation from our TA. Thanks to all for the replies.