That one is probably a float chamber vent line, intended to go to a charcoal canister on a car engine, not a boat. Leaving it open might allow gas fumes to accumulate under the engine cover and present an explosion hazard. Marine carbs have a different vent system to prevent vapor buildup in bilges or under engine covers.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!