I have craved seafood through my pregancy BIG TIME things like lobster, crab and prawns and so did my mate with her 2 pregancies and since it been christmas prawns are everywhere, she had no issues with her 2 pregancies with the seafood (one of which was a multiple) I think its common sence and all up to yourself and your body and what you want to put into it.
If you buy it somewhere fresh and clean its a good thing and steer clear of the deep sea fish as mentioned, I dont see harm and neither did my midwife when i asked her if it was safe for me to eat them