Stalk, stalk, stalk the website day and night, I have gotten them like that, even when we were silver.
If that doesn't work, as soon as you board, go to Port Aventures desk and put your name on the waitlist, I have gotten then that way too.
We are platinum and I don't always get lucky, sometimes Concierge grabs them all up before I get my chance.
Serenity Bay cabanas are much easier to score than Family beach, but I have gotten both in the past.
Being on the smaller ship also gives you a better chance.
Good Luck!