Water and Unsweetened Iced Tea, just bring your packets of Stevia, the main dining room does not offer stevia, there are 1 or 2 stations in the buffet area on Carnival that does carry the green stevia packets. Royal does not carry Stevia, but they do offer the ones that causes cancers like sweet and low and equal, light pink, blue and yellow packets.
Note that the Sugar Free Cakes and Desserts are not Diabetic friendly as they are made with wheat flour and Sucralose (Splenda) and Splenda does spike blood sugars in some folks.
You can draft up and print out a Keto Custard Dessert and hand it over to the Matre'D, they will be happy to accommodate you so that you can have desert with everyone else.
Also note that if you are suffering from some type of Neuropathy, nerve damage in the hands and feet then stay far away from anything that has Aspartame especially diet sodas.
Fred