[jsiva]

The water molecules aren't chemically bonded to the ions, but they are "bonded" by intermolecular forces. Although weaker than a chemical (intramolecular) bond, the intermolecular forces are still strong enough to "bond" water molecules to the ion. So either only water molecules not "bonded" to ions can pass through these channels, or the pressure differential across the channel can free the water molecules from the ions.

[infinite8s]

It's not quite a pressure differential, but a chemical potential differential (but thinking of it as a 'pressure' gives a useful mental model).

[notfish]

It’s force per area so it’s definitely a pressure, the difference is just that the driving mechanism is chemical potential instead of a difference in number of particles.

[contravariant]

Tough a system put into contact with a resevoir of a constant concentration should try to increase in volume until the concentrations on both sides are equal.

It's probably the 'constant concentration' part that is confounding the two. It is connecting number of particles with volume, whereas the canonical ensemble has them as separate terms.