I've never seen anyone tossed for wearing sandals. I've never even seen the choice of footwear questioned. Most sandals have more substance than some high fashion shoes!
From the Liberty Of The Seas Cruise Compass last month on a casual night in the dining room: "Please note that bare feet, short pants and tanks tops are not permitted at dinner."
For the always casual dinner in the Jade and Windjammer Cafe: "Please note that bare feet and tank tops are not permitted in the venues for dinner".