You guys are right to be scared... I'm speaking from experience. I had thru-hulls in similar shape to those pictured. I was also a little concerned, but decided I'd "test" how they worked while my boat was in the water.
. I Opened them all the way, then closed them. I put a final turn into one and the entire cover popped off, which let in a LOT of water very quickly. This happened because I put in a little to much force tightening the valve. Scary...
Anyways, I've since replaced with proper seacocks. It's the first time I've done that type of jopb, and it took me half a day to do two. Just make sure you have some big wrenches ("channel-Lock" worked very well for me) and a crowbar to hold the thru-hull while you tighten things up. I was surprised at how easy it was.