If your boat has any through hull fittings below the boat's static waterline, and it most probably does, then you also will have some anti-siphon valves like the one shown here installed:
These little fellows are among the most neglected service items on your boat. they really don't help to make anythig actually function and they don't offer any improvement of a system's performance. All they really do for you is to eliminate the risk of your boat sinking, either at the mooring or while you are sailing along enjoying a fresh breeze and a beautiful sunny day. Nothing big, they just help to stop the risk of water back siphoning up through your head or flooding out your engine with sea water.
So, how come they get ignored? I'm not sure, because as far as I'm concerned, these valves are among the most important on your boat when it comes to keeping water out of the wrong places.
So, as I do almost every year at about this time I present my little rant about servicing these valves so that you don't have to worry about the worst happening when you least expect it. All you need to do is close the seacock at the hull end of the hose and then un-screw the small cylinder at the top of the hose elbow. Inside you will find either a small rubber or plastic check plate that can become encrusted with calcium over time. Wash it out with some vinegar and warm water and re-assemble the valve. You're done. If it appears damaged, get a new one.
Once cleaned you won't have to worry about the valve sticking so that it can't vent and prevent a siphoning action from forming when you least expect it. Now was that hard? Way easier than re-floating your sunken boat....................