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Does the net provide a bonus to the "parry" subaction, or since it has an extra effect the talent is designed to NOT provide the size bonus to the parry action when using the net ?

The way that I understand it, cast net is used as something of a substitute for parry. It also adds the possibility of entangling the weapon. I'm pretty sure it is designed to not add a size bonus. However, if you were using the parry talent from a second discipline (since the boatman never gets parry) then I would add the size.

Cast Net is an alternative to Parry, correct. You don't get the Size bonus, but instead get the entangling chance.
If used for parrying, a net would provide a bonus, as it has a Size. But you wouldn't get the entangling effect.
Because you can only use one defensive action against an attack, you got to choose (this is also why Boatmen don't get Parry)