A lot of opinions here, mine is pretty simple:
Have fun with it! It's already paid and I also wouldn't waste my hard earned money.
No expectactions should be the theme. You go there to relax on the cruise, drink, have fun, meet people.
If stars allign, you're gonna have sex (with her or other women). If not, you can still have fun.
Not everything in this life is about sex! Get tan, swim, relax and enjoy the beauty of the whole cruise.
But I can't stress this enough: the less expectations you'll have, the better you'll feel and the more enjoyment you'll get out of the whole deal. Have fun, my friend!