I am assuming it is p&o you are going on? If yes, eat at the al carte restaurant, and cafe's on board, not the buffet restaurant. Yes, there is still a food risk, but nowhere near as high as where everyone could possibly contaminate food..

Honestly though, with what I have read of your pregnancy, I wouldn't be going, even for a weekend. There are huge call out fees for medical fees on board, and I don't think travel insurance covers pregnancy, and you are not covered by the Australian Medicare system.