I'm not water so I can't test.
Simple test is just do a couple fight against kakashi to get the average poison damage
remove only the scroll from the attacking ninja and repeat
remove only the scroll from water main and repeat
remove only the weapon from water main and repeat (if poison is based on the damage dealt, this would be the factor)
and see which one(s) impact the poison damage.

I'd say do 10 runs with each, record the first instance of poison damage in each, and take the average of the 10 for each set for comparison. If your weapon/scroll is of reasonable level, you should be able to tell a difference easily.