Cruise casual for boys & men would be collared shirts & slacks, and closed-toe shoes.
If your dressy flip flops could pass for regular sandals, they'll be fine. If they're actual beach wear meant only for use by the water, then not so much.
No one will police any of this at the MDRs, as a PP indicated. However, there is a difference between what you can get away with at an MDR, and what is really appropriate there.