Are you leaving from S'ampton? If so just pack the champers in your luggage and open in the cabin or pay corkage on the ship in the restaurant. I have to drink loads of water so I take 5 ltr bottles on board at s'ampton (just put a luggage label on them)as they are so expensive on the ship. Squash is no problem. Remeber to ask for diabetic foods/cakes/jam etc.
You do not need to take water on board as the water in your cabin bathroom is drinkable, no problems at all taking other drinks on board.
I'm diabetic too, and took water and squash on board and stored them in the fridge alongside my insulin! If you like fizzy drinks it is really easy to get them from the bars on board too.