Bioshock Infinite Vigors Guide: Undertow

Undertow: (Press/Hold) $150 ($125 from Vending Machine) (Emporia if not found)
Cost: 31/31
Damage: 0/40-150 on impact
Effect: 3s Stun/3s Stun

Upgrades:
Mod: $306 (Emporia)
Boost: $1143 (Downtown Emporia)

While this attack does not do any damage on its own (unless you use the alternate fire) it gives you some distinctive options. Things hit with Udnertow will either be pulled toward you or thrown away from him. This gives Booker a distinct advantage if he is near a ledge and low on Salts. It presents a quick option that Bucking Bronco does not. Using this on the Zealot, Fireman or Beast will give you the option to quickly grab them and throw them over the railing. This gives you a quick out if you are low on health or do not care to fight them. If you use this attack against mechanical foes (Ex. Handyman, Automatons etc.) it will do damage in addition to stunning them.

The Alternate Fire allows Booker to pull any opponent over to him. The range on this alternate fire is incredibly long. It will work on even distant Snipers who you cannot get a good bead on otherwise. It also has the added bonus of doing a quick slam attack when your target reaches you.

Upgrades:
The first upgrade, the Undertow Mod, allows Booker to grab one additional foe at a time. This makes it better against a small group of enemies than it was before. It does have its own advantage especially when paired with the second upgrade or the Shock Jockey Vigor.

The second upgrade, Undertow Boost, doubles the effective range of Undertow. This means that with the Alternate Fire, if you can see it, you can grab it. It also makes it more effective against smaller groups.

Combos:
There is only one really effective combination here. Simply mix Undertow with Shock Jockey. The effect is identical to blasting water around the target. This is give you a way to instantly finish off a group of foes when you only have one shot of Undertow and a little extra salt. After that just watch those affected drop like flies.