Update
The secret passive is that the bodies of small champions are gone when the shark eats them
and also players have noticed
Fizz turns a dark shade of blue when in the river, http://lol.zone.gamebase.com.tw/zone/lol/skin_view/fizz_full?lang=en#stop
any red wanna confirm?Double Hidden passives? THE COMMUNITY MUST KNOW!

Old
Could we get a little hint about what it is

it took us about 2 months to find out Leona's sunlight passive vs Sunglasses.....

Most likely its something to do with the environment

Clues
Apparently its pretty obvious, i hope so :S
does not involve 2 fizzs but it will occur more likely

Possible things
-try surfer singed with Fizz
-try angler jax
-try vladimir pool + Fizz Ultimate
-related to river in SR?
-Try Trident pantheon + Fizz

What did leonas turn out to be? I am going to guess he does bonus damage to fire based champions?

Fizz's is way more obvious than Leona's.

But you may or may not see it, depending on who you fight.

Though, considering there should be a Fizz on both sides in every game, your chances are high.

He can pass through other fizz's?

inb4 fizz UP and WhattayaBrian makes riot buff him

I'm assuming if you're standing in the river in SR, Fizz will randomly eat a fish.
Kind of like Skarner's "Pokemon" reference?

hinting that it's a Fizz vs Fizz only passive?

Shark fight? SHARK FIGHT!!!

USE SPLASH ATTACK!!!!i!!

bet on it