What you can do is to make as clear as possible what it exactly is what makes you nervous.
1. You do not know them: You will get to know them soon enough, as said, people are only strangers the first night.
2. They might not like you: That's then too bad, you can't like everyone, but always try to be pleasant and look for something you have in common.
3. You run out of things to talk about: You can think of topics on forehand. But there are always safe options like: where are you from, what do you for work/fun, the ship, the food, the entertainment, what did you do today/what will you do tomorrow, excursions or other activities you did, vacation, hobbies, your children etc. Just stay away from controversial topics like politics, religion, finances.
For everything keep in mind: dinner last 1-1,5 hour. That's 7-10,5 hours out of the whole cruise. You do not have to see them after this cruise ever again. Don't make it bigger than it is.
And lastly: It could also turn out alright, for all you know, you are about to meet your new best friends for life!
