On a Carnival Cruise, the answer depends entirely on the specific evening and the dining venue you choose. During "Cruise Casual" nights in the Main Dining Room, Carnival's official policy generally lists "beach flip-flops" as prohibited attire. While dressier leather sandals or high-quality flip-flops might sometimes slip by, the thin, rubber thong-style shoes meant for the pool are technically not allowed. On "Cruise Elegant" nights, the rules are much stricter: flip-flops of any kind are explicitly banned in the Main Dining Room and specialty steakhouses like Fahrenheit 555. If you absolutely want to wear flip-flops for dinner, your best bet is the Lido Marketplace (the buffet), where the dress code is much more relaxed and footwear is required primarily for safety and hygiene rather than style. However, even at the buffet, you should ensure you are dry and wearing a proper cover-up. For a hassle-free experience in the formal dining rooms, it is best to pack at least one pair of loafers, boat shoes, or dress sandals to ensure you meet the minimum "casual" requirement.