The Water Tornado is a Hardmode trap sentry weapon. Of note the water tornado only causes 5 frames of enemy immunity hit. Enemies who don't resist knockback are sucked into the water tornado on hit. It has a 33% (50%) chance of being dropped by Duke Fishron.
Its best Modifier is Ruthless. The Mythical modifier provides the widest array of stat bonuses, but these primarily affect the initial summon rather than the sentry's damage. Additionally, sentries cannot deal critical hits under normal circumstances. The only significant advantage a Mythical Water Tornado has over a Ruthless one is knockback.