Most of the mussels we eat are farmed not far away. The wild ones tend to be very gritty and have too much 'beard'. Most people who eat wild mussels hang them in clear seawater off the wharf in hopes they will clear a lot of the grit before being cooked.

Yes 'dodgy' is a scary word, but in fact works fairly well if you are familiar with what a good normal mussel (or clam) looks like, as long as you didn't get them from a zone restricted for disease.

The most dangerous aspect of cooking clams and mussels is ignorance. Every mollusc related intestinal disaster I've witnessed began with a cook who couldn't tell a dead clam from a live one. If they are open a bit and don't close when touched, they are dead. Don't cook them. As they cook, the shells open a little. If one doesn't open - don't eat it.