Both Bangor and Carrickfergus marinas are fine for visits to Belfast but for a different experience head for the small marina close by the Titanic Experience. This is very convenient for the City too. You have to report to the VTS on the way in (details in Reids I think) but it is very straight forward and they are helpful. The marina is close by a night club which can be noisy especially at weekends. All this info is a few years old. May head that way this year on way to Scotland.
+1 for the titanic marina.
It is nice setting and very central to the city. usually see quite a few visitor boats using it.
The Marina is 5 minute walk from city centre, with lots of nice eateries, a couple of michelin starred if thats your thing, plus lots of others for all budgets.
If you can, book OX for lunch, and have a glass or two in the merchant, see the titanic centre and take the ride. SSE arena is on your doorstep, so maybe catch a show, or visit the concert hall.