In order to play the legendary River King, you must refuse to succumb to Culinary Degradation. What this means? Well, I'll tell you!

In laning phase, start Tongue Lash. The amount of poke it does is absolutely insane, not to mention the slow it gives allows you to catch up, deal some damage and activate An Acquired Taste, stun him and continue to slap the hell out of them with your Tongue.

When your opponent retreats to tower, hopefully you'll have An Acquired Taste Up. If you do, devour them, spit them out of its range and continue to slap them.

In Team Fights, you must tank for your team. Stick close to your ADC and Devour them when they are stunned or in immediate danger. If there is only one threat on the enemy team, Devour them for the whole duration you can. It'll Be a 5v4 And most likely a won outcome.

During team fights or even duels

In Late game, if contesting objectives. Use Abyssal Voyage, an amazing ability that allows you to teleport to a location near instantaneously, and with a friend too! Your opponents will be notified of the teleportation, but it is a short time to react, and you and your ADC or jungler can surprise opponents doing objectives or a flank.

Pros
-Strong 1v1
-Very Good Early Game
-Strong Team Fighter
-Damage Sponge
-High survivability
-Hard and Soft CC
-Good Distance Traveler
-Excellent Pursuit

Cons
-Very Immobile
-Vulnerable to CC
-Doesn't Scale very well
-Abyssal Voyage is able to be cancelled