Do dead bivalves always float in water?
Do they close is a better predictor. 

before cooking tap the bivalve if they remain open do not cook or eat them.  Unresponsive bivalves of any kind not safe to eat are dead and there is no way to tell for how long. Whether they open during cooking is meaningless for mussels they can stay locked tight even if healthy and freshly killed.  Smell is also a good predictor never eat shellfish that smell off.

The differences between wild and farmed mussels makes this even less useful, wild mussels may be filled with sand making them heavy even if dead. Of course if a mussel is comes pre-cooked only smell is useful. 

float vs sink is not really useful since it relies on how tightly sealed a bivalve is and whether it trapped air when it closed. A air seal is not a guarantee with many bivalves, mussels included, nor is trapped air. It is just not a useful metric.