I'd imagine that if the fish was properly cooked, the cells would most likely be dead, and so no, they couldn't multiply and cause cancer. If the fish was raw however, I'm not altogether sure. I'd think that most cells would be killed in the stomach, or recognised as foreign and destroyed by the body the same as any pathogen, but I can't be sure, so don't take my word for it.
If you were to get cancer from eating a cancerous fish it would not be from the cancer cells of the fish but more to the extent to which phobos and matthyaouw said. The reason for this is that the cells of the fish undergoing mutations are not of your body and therefore upon entry will be attacked as a foreign invader. If it was possible to spread throughout the body its chances would be very low, but because they are not of our body they cannot produce cancer in humans. Although your question was a very good one. I would not have thought of that. Good job Aki
oh, I now get the joke in "Bruce almighty" that jim says "perhaps you put to much wasabi on your sashimi" to a guy who had just been on fire. Actually it think it was in the deleted scenes but I am not certain.