Scuba Equipment
One dive bag containing scuba equipment, other than an empty dive tank, may be accepted as part of your free baggage allowance. An empty dive tank will not be included in determining the free baggage allowance and will always be subject to a $80 fee. Scuba equipment accepted in excess of the free baggage allowance will be assessed the appropriate fee based on destination. Scuba tank must be empty. If the spear gun has a CO2 cartridge, the cartridge must be empty.

Northwest does not accept liability for loss, damage, or delay of scuba equipment. Excess valuation insurance may not be purchased for transport of scuba equipment

I see that original post was in January and it is amost eight months later so they must have changed their polidy.  Your dive bag can count as part of your free baggage allowance,  but the tank will always be subjected to an $80.00 fee.  I know I've to talked to Capt Darrick about carrying tanks on trips and he never does.   so I guess its safe to say if your bag is not overweight and your not carrying a tank, your baggage will be free.