Even for those of us who have not had problems with dizziness or balance beforehand, there is some residual effect from a cruise which gradually fades. After my first couple cruises, I noticed the sensation - mild, but still noticeable - for about a week afterward. Since then, as I have gone on more cruises, I think my inner ear has adjusted better to the change, and the feeling only lasts for a couple days. A mild form of what you are experiencing is quite normal.
Since you had problems before your cruise, you should probably talk with your doctor about it. Perhaps what was bothering you before is not completely gone, and was just "triggered" again by being on water. In any case, it's probably a good idea to get checked out.