Kunkka is a hero with a huge teamfight presence, albeit one that requires some finesse. This guide will hopefully make it more clear how you should approach the game with the Admiral himself.

In my opinion at least, Kunkka can work decently well in any position, although he prefers to be a core. Regardless of the position he's played in though, his main role will be initiating/counter-initiating ganks and teamfights, and pushing/counter-pushing with Tidebringer.

Now updated after 6 years or so!

Quick picking guide:

Strong against:

1. Heroes with lots of smaller units (illusions/summons/captured units).
2. Heroes that rely on a single escape (Anti-mage, QoP, etc).
3. Heroes that rely on strong burst damage in teamfights (due to boat buff, includes heroes like QoP/Lina).
4. Low health heroes (notably Enchantress), due to cleave.
5. Weak ranged laners or melee laners (due to huge Tidebringer harass).

Weak against:

1. Tanky heroes that can survive your cleave + boat combo (Underlord, Ogre Magi, Wraith King, etc).
2. Heroes that can turn other heroes into the aforementioned tanky heroes (Treant, Winter Wyvern, Dazzle, etc).
3. Heroes that can easily disrupt your combo (Silencer, Lifestealer, Riki, OD, Juggernaut, Euls-builders, etc).
4. Heroes that shred tanky heroes (Lifestealer, Bloodseeker, Ancient Apparition, Necrophos, etc).
5. Strong ranged laners (Viper, Necrophos, Clinkz, Drow, etc).