The exhaust after the water
injection is still pretty warm. I would only use hose made for exhaust. It's heavy duty, thick walled, black stuff that will stand up to the heat, gases, etc. This is not an area where you want to cut corners.
One thing you should be aware of. If the exhaust hose ruptures or even leaks
a little it will fill the boat with poison gas. A rupture will pump water into the bilge
much faster than you would ever imagine.