The Fuel Tank would eventually collapse and the Carbs would have to fight against a high Vacuum (Engine would run Lean).
And if the Carbs have low Pop-Offs, the Fuel would drip into the Carb Throats, while the Engine is shut-off.
That's why in Yamaha's, the Fuel Tank Check Valve is a VACUUM BREAKER. It only allows Air IN to the Tank. It is not a Positive Over-Pressure Relief.
And that's why the Check Valve ARROW must point to the Fuel Tank because that is the proper flow direction.
P.S. It is not safe to ride a Jetski with an open Tank, as the Fuel Vapors could ignite and cause an explosion. This should be obvious.
If something is pressurizing the tank when it has a check valve, why wouldn't it pressurize a tank with no vent at all?