Freshwater Cephalopods?

For anyone who puts enough thought into animals and environments (particularly if they think about cephalopods), they will eventually wonder why there are no freshwater cephalopods. If you think about   other families of mollusks you know that they dwell in water both salty and fresh water, as well as on land. So why not cephalopods? They are most of them top predators in their ecosystem.

The simplest factor that I could think of and find was how much salt there is in the water. The reason for this is simple. Freshwater species of fish actually conserve salt, while salt water species expel it. A more physical explanation for this is that cephalopods don't have a sodium pump.

Now for a crazy theory of my own: My theory is that even if cephalopods did develop the proper organs for freshwater life they could never move in the ways that made them top predators in the first place. The octopods, squid, and cuttlefish could never keep up with the rest of the creatures in the rivers because saltwater makes you lighter. Also the nautilus would be robbed of its controlled buoyancy ability (once again because you are more buoyant in saltwater. Please keep in mind I am neither a marine biologist nor have any proof behind what I say, as it is just a theory.

Sodium pump: