Preggo craving

I got the strangest cravings for sum deviled eggs and Tuna fish. Sum I don't eat but I want it!! One girl told me at work tuna isn't safe to eat. Now I'm scared but I want sum Tuna. I thought about buying sum out of Walmart and jus making sum. Is it safe to eat? The girl said sum bout too much Mercury. It sounds funny but I wanna ask you guys