Keep checking and make sure to get on a waiting list as soon as you can. I don't know if you can phone to get on the wait list or if you have to do it on the ship. If you have to get on the waitlist on the ship, try to get as early a PAT (port arrival time) as you can and make a beeline for Guest Services.
The best way to get a cabana is to cruise in concierge. But people do luck out. Just remember that the cabanas on Serenity Bay are cheaper, but it's an adult only beach, so if you have little ones, they can't join you. That also makes those cabanas just slightly easier to get.
Is it worth it? That depends on you. If you're going to be on the beach all day and you like the respite from the sun, then maybe it is. I've heard it described as costing the same as any other excursion.