Once the Sea Witch gets to level 6, she can effectively destroy enemy buildings with her Tornado skill.
With it, she can become at least partially decent for melee combat.
Basically, the Sea Witch uses mana as hit points with this ability, allowing her Intelligence to act for her instead of Strength. She becomes even more effective for combat with the Mana Shield. She should target one of the most powerful enemies first, to make sure it is hit by the lightning. The Sea Witch is also great at delivering high damage to enemy Heroes or clumped units with her Forked Lightning skill. By the time the other player realizes their hero is almost dead, it will be too late and they'll be too slow to run away. The Sea Witch is a great Hero and unit killer, mainly because of her Frost Arrows' slow effect, which prevents enemies from fleeing from battles as easily.
While she isn't a great melee fighter, the Sea Witch is useful for ranged assaults and staying back behind the melee units in battle. The Naga Sea Witch is an exceptionally powerful Neutral Hero. These scaly denizens of the ocean have often been associated with the coming of prodigious storms, but few have actually seen them rise from the sea and lived to tell the tale.