Good grief, I have been plagued by this on two boats now. The physics of putting a water-cooled engine at or below the waterline are dodgy to say the least. An option to make the vented loop "bulletproof" is not to rely on a spring-loaded plunger at the top of the loop, but to directly attach a length of hose from that barb directly up, if possible. Gooseneck the thing on deck if you wish. In other words, no "valve", no problem. No moving parts.
Same with the fuel and the water vent lines. Putting them at the waterline without valves is asking for trouble, just as putting an exhaust hose without the biggest riser you can rig (or even a waterlock aft) is trouble as well. I've always liked the idea of guys who've made a a "vent manifold" and run the lines up a stanchion or up a mizzen mast. Air gets in and does its pressure thing, but water can't.
Calder's books do a good job of explaining this, and will help you keep the sea out of your water and fuel tanks as the principles involved become clear.