Additional info - sometimes not even the water will save you from horrendous leg cramps (usually at night or when lying down) - like screaming so loud the neighbors are thinking of calling the cops or an ambulance - what happens is lactic acid settles in the mussels and needs to be displaced. Drink some brine (pickle juice) when you can get to it or have some one bring it to you - then drink as much water as you can hold.
The clams and oysters are really pissed...they want some acid, too.