I went on one of these in August this year. There were salads including stuff like tomatoes, which I ate with no ill effects. None of our group had any ill-effects, and we all chose different dishes at every meal. The main problem with the food was the fact that there was a chef preparing individual main courses to individual preferences in the middle of the buffet, and huge queues built up at every meal time.
Just think about it - if these boats were getting reputations for rotten food/food-poisonings, the tour companies would stop using them. It is very much in the tour companies' interests to ensure that their travellers are not made ill by food. Their reputations depend on having happy tourists going home telling everyone what a good time they had. Also, if travellers get ill, the tour-guides have a terrible time sorting out hospital visits, treatments, etc.