Well, i will keep this short. Yes, you need to flush your engines by hooking them up to a hose and running them. If you have the owners manual, it should say where to hook up the hose. I am not sure if yours is a twin engin or a single, but if its a twin, you need to do each motor indepently.
Hook up your hose, fire up the engine, turn on the water, rev it up a bit(i hold my between 3k-5k RPM for about 20-30 seconds), let it idle for a min, kill the hose, kill the motor, detach, hook up to motor 2, and repeat. After that, clean your boat and rinse out the engine bay as well. I use WD 40 over the summer on anyting metal. If going for a longer storage, for for something thicker. One other step i do is at the launch ramp, after its out of the water, i fire up the engine and give it a quick blast, to get some of that salt water out before i hit the road.
As far as how salt water can damage your boat? Well, almost anything that is metal that you dont clean/protect, can and will rust. There are a couple of bolts on the inside of my skis that i didnt see after the first run, and they started to show signs of rusting, sprayed those as soon as i found them, and now i try to hit every little area that i can. Salt water destroys boats if you dont clean them afterwards.
There may be some better responses from some of the boat owners, i am kinda trolling over here in your territory.