Late December, January and February are high season (most expensive) for Caribbean travel. $565 per couple all inclusive in High Season sounds about right. If that price includes air then you are getting a deal.
We usually cruise during February and our Jamaica trip is usually late October or November and the rates are better. We went to CTI 2x in August/September and it was <$300 per night not including air which was a bargain but we were taking a chance during prime hurricane season but the weather was spectacular both times.
I know that Sandals is much more than Couples because that is where we started and GLN is not what it used to be and RIU allows children and the food is only "ok" so we like couples.
The rooms are not glitzy and we like that, you can get that in the mega resorts in Cancun