Jet ski plumbing help please

[Fuges]

Just finishing a 3m jet boat with Kawasaki 900 running gear and have forgotten how it was plumbed up.
Can anyone tell me where the hose off the 3/8" outlet from the bottom of the exhaust expansion chamber is meant to go too? is it just out the back of the hull? if so what stops water from going back in when parked up,
Any help greatly appreciated.